Fix OAuth 2 handler to support retrieving parameters from other places; various OAuth client (such as the Go library) send the values in the request body or even the Auth header

util/headers.py

@@ -0,0 +1,16 @@
+import base64
+
+def parse_basic_auth(header_value):
+  """ Attempts to parse the given header value as a Base64-encoded Basic auth header. """
+
+  if not header_value:
+    return None
+
+  parts = header_value.split(' ')
+  if len(parts) != 2 or parts[0].lower() != 'basic':
+    return None
+
+  try:
+    return base64.b64decode(parts[1])
+  except ValueError:
+    return None