FormHttpMessageConverter should be able to read Multipart responses

[fpicchi]

FormHttpMessageConverter currently doesn't support reading multipart responses: it can only write requests. Unless I'm mistaken, this means that RestClient can't be used out-of-the-box to make http calls to services which return a multipart response.

Considering that:

• Controller endpoints support reading incoming multiparts (via MultipartResolver)
• Reactive's WebClient can read multiparts
• Spring Integration (HTTP module) has a HttpMessageConverter which can read multiparts (but sadly AFAIK it cannot be used in RestClient because it requires a special instance of HttpInputMessage to work)

I think FormHttpMessageConverter should support reading, or there should be an alternative HttpMessageConverter which can be used for these type of responses