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.
Update valid calls per Filepicker docs
Fix content type for store_url call.

For some reason (I have an email in to Filepicker to find out why),
the Filepicker API does not work with a content-type of
application/x-www-url-encoded-form. RestClient forces this
content-type when a hash is provided as a payload. This commit changes
the payload and specifies a content type of text/plain.
Merge branch 'policy-encoding'
Merge branch 'store-url-fix'