don't prepareOptions twice in `renderFile()`

Doing it twice, once in `renderFile` and then in `render` makes it lose some options.

Fixes sindresorhus/grunt-sass#95

I also made the test actually test includePaths as the current test was faulty.
Merge pull request #334 from sindresorhus/fix-includepaths-renderfile

don't prepareOptions twice in `renderFile()`