System.Net.Http.HttpResponseMessage.ReasonPhrase can return null on core

[Tratcher]

There was a recent regression with WinHttpHandler where HttpResponseMessage.ReasonPhrase now returns null rather than string.Empty if no reason phrase was specified in the response message. This diverges from the HttpClientHandler/HttpWebRequest behavior in the full framework that always return string.Emtpy.

@davidsh

[stephentoub]

cc: @justinvp... Justin, was this caused by your recent optimizations around the reason phrase?

[davidsh]

This was caused by @justinvp changes to the WinHttpHandler response parser

[davidsh]

PR dotnet/corefx#3199 caused this regression.

[davidsh]

And we should check dotnet/corefx#6322 as well (CurlHandler) to make sure it has correct behavior.

[davidsh]

As part of fixing this, I will be modifying our Azure test server endpoint:
http://corefx-net.cloudapp.net/StatusCode.ashx

to support sending statuscode but no description.