Almost everyone using this API actually wanted String instead of a
ByteBuffer anyway, and there were a bunch of slightly different ways
clients would convert to String.
Let's just cut out all the confusion and make it return String. :^)
These are supposed to be interpreted caselessly so let's just use the
case insensitive traits throughout. This means we'll understand things
like "Content-Length" even when they send "content-length" etc.
