Verify correctness of endpoints with file parameters

[macisamuele]

While working on #90 I was having troubles on writing a simple test that sends a file to the mock server.
This was mostly caused by the fact that the mock server is receiving the toString representation of the RequestBody and not its real content.

At this point I'm not 100% sure that the current plugin is able to generate proper retrofit APIs that allow file uploads.

The scope of this issue is to track the effort needed, if any, to ensure file upload capabilities.

[macisamuele]

I do have some conclusive results over here.
I'm currently preparing a PR to address the issue and to write the missing tests of #90 .

According to the swagger specifications in order to send files the operation must consume multipart/form-data (doc). The junit-tests file endpoint does not define multipart/form-data and so the generate code does not work well.

Considering this we have two alternatives:

1. Find a way to make it work anyway
2. Stick to the swagger specifications and annotate the endpoint as unsafe so we will discourage usage of the endpoint.

I'm aiming to use the second alternative