Token instance metadata retriever refinement: process metadata if only image URL is passed to tokenURI

[vbaranov]

#3056
Fixes #3127

Motivation

Token instance metadata retriever doesn't process instances for which image URL is returned instead of Token URI

Changelog

If headers of the response from a request to token URI link contains {"Content-Type", "image/png"}, then generate token metadata JSON with token instance image only.

Chore:

• upgrade httpoison 1.0.0 -> 1.6.2

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 71fcb0f8-7c39-4aed-9c7b-91e5a72dbf60

• 5 of 8 (62.5%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 75.069%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/block_scout_web/lib/block_scout_web/views/tokens/instance/overview_view.ex	2	3	66.67%
apps/explorer/lib/explorer/token/instance_metadata_retriever.ex	3	5	60.0%

Files with Coverage Reduction	New Missed Lines	%
apps/block_scout_web/lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	1	78.57%

Totals
Change from base Build f1737080-7128-437e-bedc-841cecd52b93:	-0.01%
Covered Lines:	5715
Relevant Lines:	7613

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💛 - Coveralls