Duplicated charset when storing a RiakObject with a content type with charset [JIRA: CLIENTS-602] #549

If I store a RiakObject with a content type of application/json;charset=UTF-8, the charset gets duplicated on the stored value when read by http, ending up with this content type:
application/json;charset=UTF-8; charset=UTF-8

I did a small sample to show the issue here:
https://github.com/gfbett/RiakIssueSample