Multipart with headers per field

Currently when using Multipart fields, the headers that are sent for each fields are hard-coded and can't be changed by the caller. I need to be able to change the content-type for a specific field, declaring it as application/json.

I suggest to change the value type of the fields map inside MultipartRequest to a class that will hold the value and the headers, such as this:
```
class MultipartField {
final String value;
final Map headers;

MultipartField(this.value, {this.headers});
}
```

There isn't much left to do after that but to create a headers string from the map.

p.s.
If anyone needs it until the issue is resolved, I personally copied the entire MulttipartRequest file to a new one and made the change. I also had to copy a few functions from the utils.dart page. Not the prettiest solution, but it works.