Fixed an issue where the urlsafe_base64_encoded policy was getting reencoded.

When the urlsafe_base64_encoded policy has padding, there are
1 or more '=' characters at the end of it. Re-encoding this string
results in the `=` characters being converted to %3D, which the Filepicker
API rejects. This commit avoids that re-encoding.
Merge pull request #13 from yapplabs/policy-encoding

Fixed an issue where the urlsafe_base64_encoded policy was getting reencoded.