Bug fix when using svg-gradient in a mixin in a nested folder

Fixes #2360 

There are a few test failures right now, but I'm positive that it is due to test setup regarding a base64 encoding option. I'm not sure how to setup the test properly, but at least it is passing for the original case I was going for. Any advice to setup the test properly would be appreciated.
This is already setup to return false (path not relative) for data-uris. Any idea why it isn't working?

I don't want to add a new isPathExternal function when essentially !isPathRelative is already setup to do that job..