Separate Content-type and charset in RiakObject.

[alexmoore]

Fixes #549 (CLIENTS-602).

Currently the Java client will attach a RiakObject's charset setting to the content-type:

RiakObject ro = new RiakObject();
ro.setContentType("application/json");
ro.setCharset("UTF-16");

ro.getContentType(); // -> "application/json;charset=UTF-16"

While not 100% wrong, we should separate them.

If you save the above object to Riak, and fetch it via the HTTP interface you'll get: "application/json;charset=UTF-16; charset=UTF-16" since we duplicate the charset to the proper charset buffer, but don't remove it from the content-type.

This PR introduces a proper charset member on the RiakObject class. We will no longer attach it to the content-type, and we don't care if they set one there or not, as to not break existing code.

If this new charset member is not set it will attempt to see if there is one present in the content-type for the getCharset() and hasCharset() methods, for backwards compatibility.

Thanks to @gfbett for reporting the issue.

[christophermancini]

👍 Code looks good and tests pass.