Add a function to prepare URL for logging

Please review my code to check if the fix is correct. I think that will covers our use cases. I am not very comfortable to have a modified version of SimplePie in the project. I know that it supports plugins, maybe we should look that way to make it easier for us if we need to update the library.

See #711 
Please do not make the SimplePie library depend on a function defined in FreshRSS.
The other way around would be acceptable though.