Support UTF8 characters in URLs

After some discussions we agreed that URLs processed by content store
should always be in their 'encoded' form, ie. any special characters
should have been escaped.

The content store client libraries will actually raise an error if
invoked with a URL which is not encoded, because the urls are processed
using URI.parse.

Furthermore, rails will by default un-encode a URL received from an HTTP
request into a UTF8 string. Therefore we need to re-encode any
parameters derived from an HTTP request before making any api calls to
content store.

Reference:

https://trello.com/c/89N8zA8g/65-content-store-doesn-t-like-non-ascii-characters-in-paths
Merge pull request #6 from alphagov/support-utf8-characters-in-urls

Support utf8 characters in urls