Fix setdefault() for swob's HeaderKeyDict.

You'd think this would just work, given that HeaderKeyDict inherits
from dict and overrides the usual __thingy__ methods, but it
doesn't. It would work if you title-cased the key, but the whole point
of HeaderKeyDict is to do that for you.

Change-Id: If5c22df0690a245d1dd02fa3a52fa135235fe60d
Merge "Fix setdefault() for swob's HeaderKeyDict."