Strip empty fields in MultipartEncoder

This is coherent with what's done in requests

requests_toolbelt/multipart/encoder.py

@@ -227,6 +227,8 @@ def _iter_fields(self):
                     file_name, file_pointer, file_type = v
                 else:
                     file_name, file_pointer, file_type, file_headers = v
+            elif v is None:
+                continue
             else:
                 file_pointer = v
 

tests/test_multipart_encoder.py

@@ -1,14 +1,17 @@
 # -*- coding: utf-8 -*-
-import unittest
 import io
+import unittest
 
 import requests
-
-from requests_toolbelt.multipart.encoder import (
-    CustomBytesIO, MultipartEncoder, FileFromURLWrapper, FileNotSupportedError)
 from requests_toolbelt._compat import filepost
-from . import get_betamax
+from requests_toolbelt.multipart.encoder import (
+    CustomBytesIO,
+    FileFromURLWrapper,
+    FileNotSupportedError,
+    MultipartEncoder,
+)
 
+from . import get_betamax
 
 preserve_bytes = {'preserve_exact_body_bytes': True}
 
@@ -272,6 +275,18 @@ def test_regression_2(self):
 
         assert read_so_far == total_size
 
+    def test_empty_files_key(self):
+        """Ensure empty `fields` keys are stripped from the request"""
+
+        fields = {
+            "test": "this is a test", 
+            "empty": None
+        }
+
+        m = MultipartEncoder(fields=fields)
+
+        assert len(m.parts) == 1
+
     def test_handles_empty_unicode_values(self):
         """Verify that the Encoder can handle empty unicode strings.