Do not compile partials when already compiled

When a page, with partials, was loaded twice, a weird bug appeared: **"arguments to path.resolve must be strings"**.
In `__express.js:10`, __all__ the partials' paths were added to `templates`, to be compiled (on line 30), even if they were already compiled and stored in the cache.

I fixed it with a small `if` statement inside the for loop. Now it works correctly, at least for me (I didn't test it thoroughly)