Fix problem prefixBase option adds prefix to absolute paths incorrectly

[randing89]

When running against the options like

{
  src: ['/absolute/path/to/deep/**/deep*.txt'],
  srcBase: '/absolute/path/to/deep/', 
  prefixBase: true
}

The return paths will be prefixed incorrectly like: /absolute/path/to/deep/absolute/path/to/deep/**/deep*.txt

This pull request is addressed to solve this problem with test case attached