t5551: lower-case headers in expected curl trace

There's a test in t5551 which checks the curl trace (after simplifying it a bit). It doesn't work with HTTP/2, because in that case curl outputs all of the headers in lower-case. Even though this test is run with HTTP/2 by t5559, nobody has noticed because checking the trace only happens if GIT_TEST_PROTOCOL_VERSION is manually set to "0". Let's fix this by lower-casing all of the header names in the trace, and then checking for those in our expected code (this is easier than making HTTP/2 traces look like HTTP/1.1, since HTTP/1.1 uses title-casing). Sadly, we can't quite do this in our existing sed script. This works if you have GNU sed: s/^\\([><]\\) \\([A-Za-z0-9-]*:\\)/\1 \L\2\E/ but \L is a GNU-ism, and I don't think there's a portable solution. We could just "tr A-Z a-z" on the way in, of course, but that makes the non-header parts harder to read (e.g., lowercase "post" requests). But to paraphrase Baron Munchausen, I have learned from experience that a modicum of Perl can be most efficacious. Note that this doesn't quite get the test passing with t5559; there are more fixes needed on top. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>