Log Message:
Update py-requests-toolbelt to 0.5.1:

0.5.1 -- 2015-12-16
-------------------

More information about this release can be found on the `0.5.1 milestone`_.

Fixed Bugs
~~~~~~~~~~

- Now papers over the differences in requests' ``super_len`` function from
  versions prior to 2.9.0 and versions 2.9.0 and later.