Add dedicated multipart support for MockMvcTester

[snicoll]

While writing the documentation for the AssertJ support for MockMvc, I came to realize that there is a separate builder that is specific to Multipart requests.

There is always the escape hatch of using perform and reuse whatever was build with the existing API but dedicated support would be nice.

I am not a huge fan of a multipart as the same level as get, post, etc. But triggering multipart in the middle of the chain means we need to provide a copy constructor for the builder.

[snicoll]

Looks like merge could be used instead of introducing a copy constructor.

[snicoll]

Brainstorming with @poutsma, we agree to not make multipart() a top-level method so we'll have to test and investigate the route of downcasting the builder with a merge.