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. 
@wtfribley if you can add a `.slice(1, -1)` after the `trim()`, I can merge this in. Chrome, FF, and IE (at least 11) prepend and append string-based style values with an actual quote. Single for Chrome and IE, and double for FF. You can see that the original code used the same fixed (and SOME).