to correctly return SET() type
Add additional useful tests for autoLink()

These new tests cover an important case that could easily be missed in
the future. Links with URL's should not be re-linked.

Refs #4998
Relative paths should be created relative to pwd.

If create() is called with a relative directory and the path does not
exist, it should be created inside of the folder->pwd() and not inside
the process' cwd.

Refs #4990
Merge pull request #5001 from cakephp/issue-4990

Relative paths should be created relative to pwd.
Bump the depth of variables in stack traces to 4.

Giving more context allows developer to look at request/response data
more easily.

Fixes #5005
Merge pull request #4489 from busgurlu/patch-1

to correctly return SET() type
Merge branch 'master' into 2.6