fix invalid User agent headers

[ayrat555]

fixes

Request: GET /address/0x0000000000000000000000000000000000000000/internal_transactions
** (exit) an exception was raised:
    ** (UnicodeConversionError) invalid encoding starting at <<191, 39, 191, 34>>
        (elixir) lib/list.ex:906: List.to_string/1
        (block_scout_web) lib/block_scout_web/views/layout_view.ex:124: BlockScoutWeb.LayoutView.issue_body/1
        (block_scout_web) lib/block_scout_web/views/layout_view.ex:102: BlockScoutWeb.LayoutView.issue_link/1
        (block_scout_web) lib/block_scout_web/templates/layout/_footer.html.eex:34: BlockScoutWeb.LayoutView."_footer.html"/1
        (block_scout_web) lib/block_scout_web/templates/layout/app.html.eex:45: BlockScoutWeb.LayoutView."app.html"/1
        (phoenix) lib/phoenix/view.ex:399: Phoenix.View.render_to_iodata/3
        (phoenix) lib/phoenix/controller.ex:729: Phoenix.Controller.__put_render__/5
        (block_scout_web) lib/block_scout_web/endpoint.ex:1: BlockScoutWeb.Endpoint.instrument/4```

Changelog

• fix invalid User agent headers

[coveralls]

Pull Request Test Coverage Report for Build 547e398c-2def-485c-9d24-80ac27ae9d6f

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 80.947%

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Totals
Change from base Build 3aac4871-ae92-4dc7-9561-c8b4a46b2afe:	0.02%
Covered Lines:	5196
Relevant Lines:	6419

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💛 - Coveralls