Strip backslashes, not just forward slashes, from untrailingslashit().

trailingslashit() will now remove any forward or backslashes from the end of a string before appending a forward slash.

props knutsp, willmot.
fixes #22267.


git-svn-id: https://develop.svn.wordpress.org/trunk@27344 602fd350-edb4-49c9-b593-d223f7449a82