Escape quotes in map-serialize function

The behavior of quotes when interpolating variables is currently different between Ruby SASS and libsass -- until libsass matches Ruby's behavior, escaping quotes is necessary when compiling with libsass (and will also work just fine when compiling with Ruby).

I've opened issue [#694](https://github.com/sass/libsass/issues/694) on libsass regarding the differences between ruby and libsass when interpolating quoted strings. 
Is there a chance this method could ever be used to actually parse a query string from the URL? If so I'd rather solve it with

```js
str = str.trim().replace(/^["']|["']$/g, '')
```
If not, I should rename the method to indicate its use only in parsing information from styles.