title: How to add a Map to MapEntry in flutter dio package
body: <p>I want to send a form data to the backend using the Dio package one of the fields accepts a map instead of a String.</p>
  <pre><code>formData.fields.add(MapEntry('category', _category));
      formData.fields.add(MapEntry('license_number', _licenseNumber.text));
      formData.fields.add(MapEntry('license_issued_on', _licenseIssueDateString));
  //The line below is the one causing the error. The argument type 'Map&lt;String, Object&gt;' can't be assigned to the parameter type 'String'.
     formData.fields.add(MapEntry('address', {'address': _address.text, 'lattitude': _addressLat, 'longitude': _addressLng}));
  </code></pre>
  <p>The FormData interface provides a way to easily construct a set of key/value pairs representing form fields and their values
  How can I go around this issue?</p>