Fix bugs in verification API v2

[nikitosing]

Motivation

There was some bugs in API v2 /verification:

1. Missed String.downcase() in sourcify verification

2023-02-16T10:52:42.834 [error] #PID<0.1751.0> running BlockScoutWeb.Endpoint (connection #PID<0.1750.0>, stream id 1) terminated
Server: localhost:80 (http)
Request: POST /api/v2/smart-contracts/0x030f7c7dbd472864220bcf9e37ede1b8a3125970/verification/via/sourcify
** (exit) an exception was raised:
    ** (KeyError) key :content_type not found in: "". If you are using the dot syntax, such as map.field, make sure the left-hand side of the dot is a map
        (explorer 5.1.0) lib/explorer/smart_contract/solidity/publish_helper.ex:97: anonymous fn/1 in Explorer.SmartContract.Solidity.PublishHelper.get_one_json/1
        (elixir 1.14.3) lib/enum.ex:4197: Enum.filter_list/2
        (explorer 5.1.0) lib/explorer/smart_contract/solidity/publish_helper.ex:97: Explorer.SmartContract.Solidity.PublishHelper.get_one_json/1
        (block_scout_web 5.1.0) lib/block_scout_web/controllers/api/v2/verification_controller.ex:118: BlockScoutWeb.API.V2.VerificationController.verification_via_sourcify/2
        (block_scout_web 5.1.0) lib/block_scout_web/controllers/api/v2/verification_controller.ex:1: BlockScoutWeb.API.V2.VerificationController.action/2
        (block_scout_web 5.1.0) lib/block_scout_web/controllers/api/v2/verification_controller.ex:1: BlockScoutWeb.API.V2.VerificationController.phoenix_controller_pipeline/2
        (phoenix 1.5.13) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (phoenix 1.5.13) lib/phoenix/router/route.ex:41: Phoenix.Router.Route.call/2
        (phoenix 1.5.13) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (phoenix 1.5.13) lib/phoenix/router/route.ex:41: Phoenix.Router.Route.call/2
        (phoenix 1.5.13) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (block_scout_web 5.1.0) lib/block_scout_web/endpoint.ex:1: BlockScoutWeb.Endpoint.plug_builder_call/2
        (block_scout_web 5.1.0) lib/plug/debugger.ex:136: BlockScoutWeb.Endpoint."call (overridable 3)"/2
        (block_scout_web 5.1.0) lib/block_scout_web/endpoint.ex:1: BlockScoutWeb.Endpoint."call (overridable 4)"/2
        (block_scout_web 5.1.0) lib/spandex_phoenix.ex:151: BlockScoutWeb.Endpoint.call/2
        (phoenix 1.5.13) lib/phoenix/endpoint/cowboy2_handler.ex:65: Phoenix.Endpoint.Cowboy2Handler.init/4
        (cowboy 2.9.0) /Users/nikitosing/work/blockscout/blockscout/deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2
        (cowboy 2.9.0) /Users/nikitosing/work/blockscout/blockscout/deps/cowboy/src/cowboy_stream_h.erl:306: :cowboy_stream_h.execute/3
        (cowboy 2.9.0) /Users/nikitosing/work/blockscout/blockscout/deps/cowboy/src/cowboy_stream_h.erl:295: :cowboy_stream_h.request_process/3
        (stdlib 4.2) proc_lib.erl:240: :proc_lib.init_p_do_apply/3

2023-02-16T11:36:45.793 [error] #PID<0.1686.0> running BlockScoutWeb.Endpoint (connection #PID<0.1685.0>, stream id 1) terminated
Server: localhost:4000 (http)
Request: POST /api/v2/smart-contracts/0x030f7c7dbd472864220bcf9e37ede1b8a3125970/verification/via/multi-part
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in anonymous fn/2 in Explorer.SmartContract.Solidity.PublishHelper.read_files/1
        (explorer 5.1.0) lib/explorer/smart_contract/solidity/publish_helper.ex:111: anonymous fn("", %{}) in Explorer.SmartContract.Solidity.PublishHelper.read_files/1
        (elixir 1.14.3) lib/enum.ex:2468: Enum."-reduce/3-lists^foldl/2-0-"/3
        (block_scout_web 5.1.0) lib/block_scout_web/controllers/api/v2/verification_controller.ex:153: BlockScoutWeb.API.V2.VerificationController.verification_via_multi_part/2
        (block_scout_web 5.1.0) lib/block_scout_web/controllers/api/v2/verification_controller.ex:1: BlockScoutWeb.API.V2.VerificationController.action/2
        (block_scout_web 5.1.0) lib/block_scout_web/controllers/api/v2/verification_controller.ex:1: BlockScoutWeb.API.V2.VerificationController.phoenix_controller_pipeline/2
        (phoenix 1.5.13) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (phoenix 1.5.13) lib/phoenix/router/route.ex:41: Phoenix.Router.Route.call/2
        (phoenix 1.5.13) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (phoenix 1.5.13) lib/phoenix/router/route.ex:41: Phoenix.Router.Route.call/2
        (phoenix 1.5.13) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (block_scout_web 5.1.0) lib/block_scout_web/endpoint.ex:1: BlockScoutWeb.Endpoint.plug_builder_call/2
        (block_scout_web 5.1.0) lib/plug/debugger.ex:136: BlockScoutWeb.Endpoint."call (overridable 3)"/2
        (block_scout_web 5.1.0) lib/block_scout_web/endpoint.ex:1: BlockScoutWeb.Endpoint."call (overridable 4)"/2
        (block_scout_web 5.1.0) lib/spandex_phoenix.ex:151: BlockScoutWeb.Endpoint.call/2
        (phoenix 1.5.13) lib/phoenix/endpoint/cowboy2_handler.ex:65: Phoenix.Endpoint.Cowboy2Handler.init/4
        (cowboy 2.9.0) /Users/nikitosing/work/blockscout/blockscout/deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2
        (cowboy 2.9.0) /Users/nikitosing/work/blockscout/blockscout/deps/cowboy/src/cowboy_stream_h.erl:306: :cowboy_stream_h.execute/3
        (cowboy 2.9.0) /Users/nikitosing/work/blockscout/blockscout/deps/cowboy/src/cowboy_stream_h.erl:295: :cowboy_stream_h.request_process/3
        (stdlib 4.2) proc_lib.erl:240: :proc_lib.init_p_do_apply/3

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
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• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools