The WebOb 1.2 support patch introduced a regression where an unknown
format= query used to just default back to plain but now returns a
400. This will break anyone that uses format=text for instance, or
anything else "invalid".
Arguably, it makes the most sense to 400 in the case of an invalid
format requested, but since it is also a backwards compatibility
break, I recommend we keep the previous behavior. Retaining the
previous behavior seems less damaging than enforcing the new
behavior.
Change-Id: I6db015b33a6f3ab239b8cb4a3562ebdba1f76590