Optimize HttpDateParser.TryStringToDate (#34860)

Optimize HttpDateParser.TryStringToDate (#34860)

* Optimize HttpDateParser.TryStringToDate

When HttpResponseMessage headers are enumerated, even though SocketsHttpHandler adds headers with TryAddWithoutValidation, the enumeration ends up validating them (that's something to be discussed and follow-up on separately).  As part of that, it validates the Date header that's required of most responses. It does so using DateTimeOffset.TryParseExact, but using a long list of allowed formats as well as options that knock TryParseExact's processing off the fast path.  Since most responses are going to contain a date using RFC1123 format (aka "r"), we just try that first, in a way that will generally result in hitting the optimized "r" code path which is much faster.

* Address PR feedback