Don't memoize status; http_status may change

Plus some unrelated fixes to the specs to make the full suite pass again.
When the server returns a 100-continue, the existing header object is re-used for the subsequent response. When client gets the 2nd response, say, 200 OK, the code (correctly) updates http_status to '200' but .status() does NOT change because it's memoized.

I guess we could have a conversation about re-using the header object vs creating a new one, but this change doesn't affect any of the current processing; just keeps status and http_status in sync.