Fix base64 images being corrupted in safe mode (issue #526)

[Crozzers]

This PR fixes #526 by not escaping certain base64 characters in URLs when an image data URL is detected.

It works by adding a step to _protect_url that checks if the URL matches the data URL format. The format allows for a MIME type, an optional base64 token and then the data. If the MIME type starts with image/ and the base64 token is present, then the charset of the URL is assumed to be base64.

An additional parameter is then passed to _hash_escape_url which skips encoding the + sign. It's important to note that the charset of the URL isn't validated in any way, so it may contain other characters as well.

[nicholasserra]

LGTM thanks!