Changed isAbsolute()-method of Folder class so it recognizes registered ...

...stream wrappers as absolute 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.

This patch will make it possible to use vfsStream (a library for testing filesystem operations) together with cakephp apps

See also: http://stackoverflow.com/questions/24188008/is-it-possible-to-use-vfsstream-with-cakephp-2-4

I appreaciate all feedback!
When using wrappers, the built-in ones seem to be case insensitive, while custom ones aren't, ie `HTTP://example.com` or `FiLe:///C:/...` are valid paths and do work fine, while `var://...` wouldn't work when registered as `VAR`.

What are the possible side effects using a case sensitive comparison that will treat such paths as non-absolute? Or let's put it this way, what is worse, not matching a valid path, or matching an actually not registered wrapper?