fix fetching latin1 encoded data

[ayrat555]

Motivation

some JSONs contain latin1 encoded data which
fails to be decoded

Changelog

• fix fetching latin1 encoded data
• process json file in token uri
• separate file for token instances logs

[coveralls]

Pull Request Test Coverage Report for Build 9bd16b64-3085-45a3-8e2e-00088e7335b9

• 8 of 15 (53.33%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 76.999%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/indexer/lib/indexer/fetcher/token_instance.ex	0	1	0.0%
apps/explorer/lib/explorer/token/instance_metadata_retriever.ex	8	14	57.14%

Files with Coverage Reduction	New Missed Lines	%
apps/explorer/test/support/factory.ex	1	92.97%

Totals
Change from base Build 60978aa7-e1f4-495a-a1b3-0569471b6af3:	0.3%
Covered Lines:	5296
Relevant Lines:	6878

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💛 - Coveralls

[vbaranov]

@ayrat555

I still see messages like this in error.log:

2019-10-15T17:43:28.593 application=indexer fetcher=token_instances [error] failed to fetch token instance metadata for {"0xbd13e53255ef917da7557db1b7d2d5c38a2efe24", 788042363036}: {:error, ""}

[ayrat555]

@vbaranov the issue was in log level. it's fixed now

[vbaranov]

@ayrat555 CHANGELOG entry is missing

[ayrat555]

@vbaranov I added a CHANGELOG entry

[vbaranov]

@ayrat555 it is a duplicate