Removing toLowerCase

Is there a reason all of the header fields are being lower cased? This actually caused requests I was making to a server I don't have control over to fail.
Fixes #161
Changed header map to use a TreeMap with a Case Insensitive comparator, which keeps the framework in compliance with RFC2616 standard, section 4.2.
Merge pull request #162 from shortstuffsushi/master

removed lower case conversion of http header fields