Fix capitalization of header for is('ajax')

The code makes a case-sensitive compare to  'XMLHttpRequest', so anyone manually setting it to XmlHttpRequest per this doc would run into issues of it not showing up as is('ajax').  I also capitalized With in the header name, since that's what jQuery sends, and case doesn't matter there (since it's pulled from the $_SERVER array which is all-caps)
Merge pull request #1453 from cincodenada/patch-4

Fix capitalization of header for is('ajax')