Existing file copy operation fails in unexpected ways

When the `dest` parameter is passed, this plugin performs a disk copy. I have a few files (heavily minified JavaScript files around 800kb) for which this process predictably fails every time, resulting in either 0-byte files or partial copies. 

Changing to the `grunt.file.copy` method seen in the official grunt-contrib-copy seems to take a bit longer but works perfectly every time.