httpboot: Ignore case when checking HTTP headers
httpboot: Ignore case when checking HTTP headers

Some servers (HAProxy) yield Content-Length in lowercase, and shim would
fail to find it.

Signed-off-by: Robbie Harwood <3f8631879978f4cea53bf1163760f1ac18f9cb08@redhat.com>

rhinstaller/shim,rhinstaller/shim,rhinstaller/shim