Ability to set the charset in postJson (or default to utf-8)

[rubenlg]

restler does a good job at parsing incoming charsets, but doesn't support declaring outgoing ones.

When I use postJson, this is the content-type line:
content-type: application/json

However, for the backend I'm trying to call, it should be instead:
content-type: application/json;charset=utf-8

Otherwise the backend assumes the JSON data is ASCII, which ruins non-English text, and emojis.

I'm happy both with having it as an option, or defaulting to setting it always to utf-8.