Bump deps and fix tests

[andrelaszlo]

Fixes #97
Fixes #63

• Bumped rack 2.0.3 → 2.2.3 - there are a lot of security fixes (see Vulnerabilities with rack 2.0.3 and rake 0.9.2.2 #97).
• Bumped rake 13.0.1 → 13.0.6 (see Vulnerabilities with rack 2.0.3 and rake 0.9.2.2 #97).
• Fixed some warnings for unitialized variables.
• There were some broken tests on master that I also fixed. They were referring to various domains, so I pointed them to a "neutral" one - mockapi.io. I have no affiliation with them.
• Changed assertion style in some tests to get better error messages. Eg: assert x==y → assert_equal x, y
• Changed header normalization to fix the issue with the Set-Cookie header. (See test_net_http_hacked and test_http_full_request_headers fail #63)
  HTTP headers are case insensitive, but Rack::MockResponse is calling headers.fetch which isn't case insensitive even when headers is a Utils::HeaderHash instance. The easiest solution seemed to be to just change our normalized header format to whatever Rack::MockResponse expects.

[andrelaszlo]

@ncr Is this useful? Would like to use rack-proxy but we need those security patches first. :) I'm available to fix feedback on Friday and any time next week.

(Sorry for the force-pushes; I needed to get author/committer to match my key or Github wouldn't verify it properly.)

[ncr]

Hello @andrelaszlo, thank you for this contribution. I will merge it and release a new version shortly :)