radosgw: fix awsv4 header line sort order.

radosgw: fix awsv4 header line sort order.

The awsv4 signature calculation includes a list of header lines, which
are supposed to be sorted.  The existing code sorts by header name, but
it appears that in fact it is necessary to sort the whole header *line*,
not just the field name.  Sorting by just the field name usually works,
but not always.  The s3-tests teuthology suite includes
s3tests.functional.test_s3.test_object_header_acl_grants
s3tests.functional.test_s3.test_bucket_header_acl_grants
which include the following header lines,

x-amz-grant-read-acp:id=56789abcdef0123456789abcdef0123456789abcdef0123456789abcdef01234
x-amz-grant-read:id=56789abcdef0123456789abcdef0123456789abcdef0123456789abcdef01234
x-amz-grant-write-acp:id=56789abcdef0123456789abcdef0123456789abcdef0123456789abcdef01234
x-amz-grant-write:id=56789abcdef0123456789abcdef0123456789abcdef0123456789abcdef01234

in this case, note that ':' needs to sort after '-'.

Fixes: http://tracker.ceph.com/issues/21607
Signed-off-by: Marcus Watts <mwatts@redhat.com>