Fix contract's method's output of tuple type

[vbaranov]

Motivation

Method getArtifact of DarkForest NFT contract returns internal server error:

2021-01-04T11:19:45.103 [error] #PID<0.1431.0> running BlockScoutWeb.Endpoint (connection #PID<0.1248.0>, stream id 10) terminated
Server: localhost:4000 (http)
Request: GET /smart-contracts/0x5cFbC3e179C48ccB3B3d35bAD6361972F2C5603B?function_name=getArtifact&method_id=8465ad44&args%5B%5D=80655615332912814281286268613438042255103875506825830758019774624527142298860
** (exit) an exception was raised:
    ** (Protocol.UndefinedError) protocol Phoenix.HTML.Safe not implemented for {80655615332912814281286268613438042255103875506825830758019774624527142298860, 996040386519514455547369964248641440971718171905442664266757499782684832, 2, 1, 1609747540, <<101, 112, 77, 187, 192, 10, 168, 130, 225, 223, 247, 185, 210, 111, 250, 61, 131, 95, 42, 109>>, 1} of type Tuple. This protocol is implemented for the following type(s): Explorer.Chain.Address, Explorer.Chain.Transaction, Explorer.Chain.Data, Explorer.Chain.Block, Explorer.Chain.Hash, Decimal, Time, Integer, Tuple, List, DateTime, NaiveDateTime, Float, Atom, Phoenix.HTML.Form, BitString, Date
        (phoenix_html 2.14.2) lib/phoenix_html/safe.ex:99: Phoenix.HTML.Safe.Tuple.to_iodata/1
        (block_scout_web 0.0.1) lib/block_scout_web/templates/smart_contract/_function_response.html.eex:8: anonymous fn/2 in BlockScoutWeb.SmartContractView."_function_response.html"/1
        (elixir 1.11.2) lib/enum.ex:2181: Enum."-reduce/3-lists^foldl/2-0-"/3
        (block_scout_web 0.0.1) lib/block_scout_web/templates/smart_contract/_function_response.html.eex:5: BlockScoutWeb.SmartContractView."_function_response.html"/1
        (phoenix 1.5.6) lib/phoenix/view.ex:472: Phoenix.View.render_to_iodata/3
        (phoenix 1.5.6) lib/phoenix/controller.ex:776: Phoenix.Controller.render_and_send/4
        (block_scout_web 0.0.1) lib/block_scout_web/controllers/smart_contract_controller.ex:1: BlockScoutWeb.SmartContractController.action/2
        (block_scout_web 0.0.1) lib/block_scout_web/controllers/smart_contract_controller.ex:1: BlockScoutWeb.SmartContractController.phoenix_controller_pipeline/2
        (phoenix 1.5.6) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (phoenix 1.5.6) lib/phoenix/router/route.ex:41: Phoenix.Router.Route.call/2
        (phoenix 1.5.6) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (block_scout_web 0.0.1) lib/block_scout_web/endpoint.ex:1: BlockScoutWeb.Endpoint.plug_builder_call/2
        (block_scout_web 0.0.1) lib/plug/debugger.ex:132: BlockScoutWeb.Endpoint."call (overridable 3)"/2
        (block_scout_web 0.0.1) lib/block_scout_web/endpoint.ex:1: BlockScoutWeb.Endpoint."call (overridable 4)"/2
        (block_scout_web 0.0.1) lib/spandex_phoenix.ex:156: BlockScoutWeb.Endpoint.call/2
        (phoenix 1.5.6) lib/phoenix/endpoint/cowboy2_handler.ex:65: Phoenix.Endpoint.Cowboy2Handler.init/4
        (cowboy 2.8.0) /Users/viktor/Documents/POANetwork/blockscout/deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2
        (cowboy 2.8.0) /Users/viktor/Documents/POANetwork/blockscout/deps/cowboy/src/cowboy_stream_h.erl:300: :cowboy_stream_h.execute/3
        (cowboy 2.8.0) /Users/viktor/Documents/POANetwork/blockscout/deps/cowboy/src/cowboy_stream_h.erl:291: :cowboy_stream_h.request_process/3
        (stdlib 3.13.2) proc_lib.erl:226: :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.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• 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

[coveralls]

Pull Request Test Coverage Report for Build de4bd037eeaf4f75d99531e9197be9d56d3b3138-PR-3551

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.9%) to 66.795%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/views/smart_contract_view.ex	4	68.0%

Totals
Change from base Build 7aa011ad9244fc5d9435c249938ef8793b604f6e:	-0.9%
Covered Lines:	2078
Relevant Lines:	3111

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💛 - Coveralls