The final known issues with GuzzleHttpRequest were corrected in T212175, and GuzzleHttpRequest is available from HttpRequestFactory if code explicitly requests it. But to gain the benefits of GuzzleHttpRequest, and ultimately complete T137926, we must switch to using guzzle by default.

Enable guzzle as the default in HttpRequestFactory (a one-line change).

This is being done as a separate task instead of as part of T212175 so that it is conveniently revertable, should issues arise.