Update isAbsolute() to recognize stream wrapper paths.

This has the benefit that the realpath() method is not applied to a
registered stream wrapper in the constructor of the Folder class.
Using the realpath() method will break the stream.

Thank "davalb" for the original patch.
Merge pull request #3773 from ADmad/2.6-folder-streamwrapper

Update isAbsolute() to recognize stream wrapper paths.