Treat header name as case-insensitive when calling getResponseHeader() on mock XHR object

This is the tiniest of trivial requests, so please bear with me.

jQuery AJAX [lowercases header names](https://github.com/jquery/jquery/blob/master/src/ajax.js#L340-L351) passed as arguments to getResponseHeader(), so I thought it would be nice if the form plugin did so for feature parity (this discrepancy actually bit me, hence the PR).