Multipart form request with fields of the same name only retains one field value

With Multipart form data, it is allowed to have multiple fields of the same name with different values and they all are submitted to the server.

A curl example of that is below (from Mailgun API documentation):

```bash
curl -s --user 'api:YOUR_API_KEY' \
https://api.mailgun.net/v3/routes \
-F priority=0 \
-F description='Sample route' \
-F expression='match_recipient(".*@YOUR_DOMAIN_NAME")' \
-F action='forward("http://myhost.com/messages/")' \
-F action='stop()'
```

Here is the test case:
```dart
test('with multiple fields of the same name', () {
var request = http.MultipartRequest('POST', dummyUrl);
request.fields['field1'] = 'value1';
request.fields['field1'] = 'value2';

expect(request, bodyMatches('''
--{{boundary}}
content-disposition: form-data; name="field1"

value1
--{{boundary}}
content-disposition: form-data; name="field1"

value2
--{{boundary}}--
'''));
});
```

The above test case fails because we store fields in a Map and so `value2` replaces the previously set `value1` for `field1`.

Start with MultipartRequest.fields and the multipart serialization path; the issue identifies the Map storage as the source of the replacement. Use the provided test case as the first check, and consider how repeated field names should be represented. Done means both values are emitted as separate multipart fields and the test passes.

Written by the indexing model from the issue text.