Use the available content type part annotation field for objects

[dzsonni]

Hello,
This is my first contribution to retrofit or any other dart open-source project so sorry for any inconvenience.

In my use case we are sending several different content-types in multipart request. So the use of the content-type header for each part is very important.
The way I got around this so far is using List<MultipartFile> and building out my own objects with content type as that seems to have worked.
I thought others might find this useful so I tweaked the generator a bit.
When content type is present instead of serializing into a simple map entry I create a MultipartFile from string and add the content-type.
This is just a PR to gauge the possibility of this being merged. I know I still have to handle the other types.

I await your input.

[sbatezat]

I think this PR broke something.
See #692