Don't delete Content-Type when formData

i use the package come a issue with react-native .. the server can not get the contet-type correct !!

i found it in code 
```
if (utils.isFormData(requestData)) {
  delete requestHeaders['Content-Type']; // Let the browser set it
}
```

why u will delete it when i manual setting it ? and the react-native not set it automatic。

can remove it ? or add a option to prevent remove ?

