URL-encoded file names break source sets

[sweber83]

When the tachyon plugin url encodes file names, wp_get_attachment_image_srcset() always returns false.
This is the case because wp_calculate_image_srcset() compares the plain file name with the url encoded url, which never results in a match.

I tried fixing the problem by url encoding the file names, too, but this leads to new problems.
What I see as a possible solution, is to not encode the file names at all.
This can be achieved by reverting 256b67e & 5db2683
I tried this in my branch here, and it seems to work fine.

I'm not really sure, if this creates any new problems.
There's another issue #23, which also has problems with the url encoding.