add validation support for multipart/form-data

[birnbera]

The WebOb returned by request.POST supports boundary delimited multipart/form-data POSTs. This change merely calls the mixed() method on the POST body when the content-type is multipart/form-data, allowing validation to proceed in cases where the body is a combination of e.g. json and binary data.

[leplatrem]

Thanks for this contribution!

Any chance you could add a small test please?

[birnbera]

@leplatrem Thanks for reviewing this pull request! Just added a couple tests.

I had to make a new service endpoint to test multiple fields in a multipart request. I left a comment regarding why, but basically it's because, as far as I can tell, there isn't any native support in colander or marshmallow for a MultiDict when it actually contains multiple values for a given key. I've seen some recipes for how to subclass list types to facilitate deserialization, but it's up to the user to pass such as schema to the view in order to get them working.

[birnbera]

Any chance this could be reviewed?

[leplatrem]

All good, Thanks!