Add Dialyxir and fix all warnings

[rkallos]

Thank you very much for Mint.WebSocket!

This PR adds :dialyxir to the project and runs it in CI.

When I added :dialyxir to this project, I saw there were warnings, so I fixed them in the best way(s) I knew how. I also cleaned some things up where I saw the opportunity to, such as in TestServer.

[coveralls]

Pull Request Test Coverage Report for Build 6d394f37eac7c2ce759e5f26480af700f023a93f-PR-27

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 14 of 16 (87.5%) changed or added relevant lines in 3 files are covered.
• 13 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 89.174%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/mint/web_socket.ex	6	7	85.71%
lib/mint/web_socket/frame.ex	3	4	75.0%

Files with Coverage Reduction	New Missed Lines	%
lib/mint/web_socket/frame.ex	13	87.16%

Totals
Change from base Build 4fbd3f5caf48d71875b2596b57eac80fa12b327b:	0.2%
Covered Lines:	313
Relevant Lines:	351

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💛 - Coveralls

[the-mikedavis]

Thanks for working on this, this is looking good! 😀

I left some minor comments

[the-mikedavis]

Thank you!