Make 'mime' field in Body optional

[halvko]

This does not seem like the ideal way to do this since Option have to be parameterized with a type, see the introduced example code for specific example.

I see a couple of alternatives:

• Add a method for unsetting the mime type
• Clearly document what mime type is the "no mime type". This should possibly be part of an example above set_mime

The first as a semver minor change, and the second is a semver patch change.

A solution to #381

[halvko]

@yoshuawuyts Does it seem allright?