(PE-5567) Fix bug if content-type header has no charset

Prior to this commit, the Java client API would throw an NPE if
it got a response from a server where the Content-Type header
did not include a charset.
Merge pull request #12 from cprice404/bug/master/PE-5567-npe-if-no-charset

(PE-5567) Fix bug if content-type header has no charset