Scalatra always returns Content-Type with charset=UTF-8

Scalatra set UTF-8 as response charset at first. So Content-Type header of response always contains ```;charset=UTF-8```.

For example, in the cases of following, I expect Content-Type header is ```image/jpeg```, but in fact, Scalatra returns ```image/jpeg; charset=UTF-8```.

```scala
get("/images/avater"){
  val bytes: Array[Byte] = ...
  contentType = "image/jpeg"
  bytes
}
```

To fix this problem, I changed ```ScalatraBase``` to set response charset just before rendering response only when it's not specified.