CopyFiles does not respect folder structure when pattern only matches files in a subdirectory

### What You Are Seeing?
CopyFiles does not correctly respect the folder structure when a pattern is used which only matches files in a subdirectory.
### What is Expected?
If I have the files:
./src/file1.txt
./src/sub/file2.txt
./src/sub/file3.dat

The command CopyFiles("./src/**/*.txt", "./dest", true) will result in:
./dest/file1.txt
./dest/sub/file2.txt

However the command CopyFiles("./src/**/*.dat", "./dest", true) will result in:
./dest/file3.dat

I would expect it to result in:
./dest/sub/file3.dat
### What version of Cake are you using?
0.26.1
### Are you running on a 32 or 64 bit system?
64bit
### What environment are you running on? Windows? Linux? Mac?
Windows
### Are you running on a CI Server? If so, which one?
No
### How Did You Get This To Happen? (Steps to Reproduce)
This repo demonstrates the issue:
https://github.com/dandezille/cake-copyfiles
### Output Log
https://gist.github.com/dandezille/f34f8f0c0fc8429be8fe85397c588856