! http(x): polish MediaType model, fix tests, smaller improvements

http:
  - remove `isContentTypeAccepted` and `acceptedCharset` methods on `HttpMessage`
     (they are not required anymore by the new content negotiation logic)
  - add `withQValue` and `withParameters` modifier methods to `MediaRange`
  - add `qValue` parameter to `MediaRange.custom` and `MediaType.custom` constructors
  - add `spray.http.MultipartMediaType` type with an additional `withBoundary` modifier
  - improve custom multipart media-type creation
     (was, for example: new `multipart/mixed`("myBoundary")
     now it's: `multipart/mixed` withBoundary "myBoundary")
  - extend `spray.http.Rendering` with support for Floats and Doubles
  - fix failing tests

 httpx:
  - remove `tryAccept(ContentType)` overload on `MarshallingContext`
  - fix failing tests