🔧 Allow proxy_* in sign_in.

This is allowed when setting the credentials *file*, however, trying to
set this inside a session used to fail.

Author: theengineear

plotly/session.py

@@ -22,6 +22,8 @@
 CREDENTIALS_KEYS = {
     'username': six.string_types,
     'api_key': six.string_types,
+    'proxy_username': six.string_types,
+    'proxy_password': six.string_types,
     'stream_ids': list
 }