Summary: Right now `FormData` doesn't allow duplicate keys and uses the last value set for a duplicate key. I tested this in Chrome:
```
var formData = new FormData();
formData.append('key', 'value1');
formData.append('key', 'value2');
var request = new XMLHttpRequest();
request.open("POST", serverUrl);
request.send(formData);
```
and the request has both 'value1' and 'value2'.
I removed the duplicate key check in `FormData`. If people want to build appending or disallow duplicate keys, they can build either on top of this.
Closes https://github.com/facebook/react-native/pull/3556
Reviewed By: svcscm
Differential Revision: D2566999
Pulled By: nicklockwood
fb-gh-sync-id: 580e52e69376ebe9693e39a386cc540802b6d94f
56fef9b622