Wrong Content-Type header for UTF-8 files

I have an UTF-8 encoded Javascript file, which I want to serve as a static asset from a GCS bucket. The problem is that the file always has `content-type:application/javascript` while it should be `content-type:application/javascript; charset=utf-8`.

I've tried using `use_magicfile = True` but that doesn't help either because the file command is called with `--mime-type` instead of the `--mime` argument which would include the charset http://linux.die.net/man/1/file.

The fix would be as easy as changing the argument here:

https://github.com/GoogleCloudPlatform/gsutil/blob/34571ffcdb82383e1a9144f9744fc8b5b97b5fdb/gslib/copy_helper.py#L1317