Allow wildcards in content-type

Currently (Rocket version 0.4.0-dev) if you use a wildcard in content-type you will get a warning like this:

warning: 'image/*' is not a known media type
 --> src/handlers/image.rs:5:33
  |
5 | #[post("/image", format = "image/*", data = "<data>")]
  |                                 ^^^^^^^^^

Using wildcard e.g. for images is needed as you do not know what the actual type of the image will be (png, gif, jpg etc.) and you want to handle all of them in the same route.