Don't die if a charset param exists along w/ mediatype

`application/json; charset=UTF-8` would fail despite being what the library
implicitely expects. Update checks and documentation to make sure we accepts
requests accordingly and reject them otherwise (even for the spec default
of "; charset=ISO-8859-1"
Merge pull request #30 from yannk/master

Don't die if a charset param exists along w/ mediatype