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.
I didn't dig into that regex earlier since the method name had me believe it was looking for things like `../../folder`, `~/folder`, or `/folder`. I didn't realize that it was doing this by checking that it was not something else.

So, I do see how it would fail for `data:image`, however `'data:image....` is getting passed in. Note the leading quotation mark. Would it make more sense to strip out that quotation mark in this method or require the caller to strip it out? Or is this an issue with the `Quoted` prototype?