Various Media Range Params Fixes

This follows up on a bug originally adressed by this pull request https://github.com/basho/webmachine/pull/21 but that I don't think was ever completed from what I can tell (I haven't been able to find any other references and I was able to recreate the issues mentioned).

2 bug fixes and a minor "improvement":

* Fixes formatting of content types that contain parameters. Originally, this would cause webmachine to crash in certain cases
* fixing above caused webmachine to crash in v3o18 instead. this no longer occurs
* It was documented in `webmachine_util:choose_media_type/2` that atoms could be used as keys in the media-types params list, this is now supported.