Letter case changes in HTTP/2

[billtomczak]

We recently found paging functions broken when cURL switched to using HTTP/2. It turns out the headers are sent with all lower case keys in http/2. Causing this to always return false due to capital-L Link::

if (isset($response->headers['Link'])) {

See:
https://github.com/recurly/recurly-client-php/blob/master/lib/recurly/pager.php#L132

[aaron-junot]

Using curl, I have confirmed that the response headers are lower case

[billtomczak]

Thanks Aaron! I have a patch in place in my application but it won't pass the unit tests because they insist on the upper-case header keys.

[aaron-junot]

If you would like to do a PR for the fix and update the unit test to accept the lower case header, I'd be happy to review and merge it!

[aaron-junot]

Fixed in #392