handle case insensitive headers when parsing for API rate limiting

[shiftkey]

Related to #2161 but probably not the full fix, given we pass around a dictionary and lookup by key, rather than following the HTTP spec and supporting case insensitive headers.

I chatted with some others inside GitHub and they reminded me about RFC 2616, section 4.2:

Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive.

I'd love to get to a place where we can leverage HttpResponseHeaders but we are very tightly coupled to IResponse in the internals, and it's this component that manages the headers as a dictionary:

octokit.net/Octokit/Http/IResponse.cs

Lines 32 to 35 in a25863f

	/// <summary>
	/// Information about the API.
	/// </summary>
	IReadOnlyDictionary<string, string> Headers { get; }

The setup is here, which converts the HttpResponseHeaders into a dictionary:

octokit.net/Octokit/Http/HttpClientAdapter.cs

Lines 125 to 129 in a25863f

	return new Response(
	responseMessage.StatusCode,
	responseBody,
	responseMessage.Headers.ToDictionary(h => h.Key, h => h.Value.First()),
	contentType);

You'll also note that headers can have multiple values, so the eventual refactoring would need to support that.

• identify other places where we TryGetValue without handling case-insensitive headers

[codecov]

Codecov Report

Merging #2175 into master will increase coverage by 0.01%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master    #2175      +/-   ##
==========================================
+ Coverage   65.83%   65.85%   +0.01%     
==========================================
  Files         545      545              
  Lines       14234    14248      +14     
  Branches      834      836       +2     
==========================================
+ Hits         9371     9383      +12     
- Misses       4706     4707       +1     
- Partials      157      158       +1     

Impacted Files	Coverage Δ
Octokit/Http/RateLimit.cs	93.61% <75.00%> (-3.89%)	⬇️
Octokit/Clients/Enterprise/EnterpriseProbe.cs	90.90% <100.00%> (ø)
Octokit/Exceptions/AbuseException.cs	70.58% <100.00%> (+1.83%)	⬆️
Octokit/Http/ApiInfoParser.cs	100.00% <100.00%> (ø)

[shiftkey]

release_notes: fixed parsing of response headers in GitHubClient.GetLastApiInfo()