Fix bug with map key caching

This PR fixes a bug which meant hashes for string keys in maps weren't be cached. This was causing massive performance issue.

```
➜  test  ./test.sh
Libsass 3.0.2

real    0m0.942s
user    0m0.914s
sys 0m0.017s

Libsass 3.0.2 with patch

real    0m0.230s
user    0m0.213s
sys 0m0.015s

Ruby sass 3.4.6

real    0m1.241s
user    0m1.179s
sys 0m0.060s
```

**Summary**
```
Libsass 3.0.2 - 24% faster ruby 
Libsass 3.0.2 with patch - 81% faster ruby
```