Add insensive HTTP header check per RFC 2616

[pjmolina]

Per RFC 2616: header names are case insensitive, so the check:

smoke_assert_headers "content-type: application/json; charset=utf-8"

should succeed when server returns the header names in any of these ways:
Content-Type: application/json; charset=utf-8
content-type: application/json; charset=utf-8
CoNTEnT-type: application/json; charset=utf-8
etc...

Room for improvement:
The current implementation and the proposed fix is not splitting the header from the value.
So header value should be checked for differences in upper/lower case.