Headers are ordered and case insensitive

Merged `OrderedHash` with `Headers` so that headers will be case
insensitive. Thought I did that back in issue-33 branch but did not
:smile:

Also removed previous `OrderedHash` stil cited in documentation that
needs to be regenerated
Display#show_headers must be case insensitive

Headers match must be case insensitive everywhere, maybe we should put
this logic in `Headers` class?
Extract custom matcher in spec helper