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.