Fixed Mustache getHelper() so that it doesn't need second argument

Mustache getHelper('helpername') would not work unless you supplied an empty Observe/Map as second argument.
Merge branch 'patch-1' of github.com:jondubois/canjs into jondubois-patch-1

* 'patch-1' of github.com:jondubois/canjs:
  Fixed Mustache getHelper() so that it doesn't need second argument
Add test for 1178.

`options` parameter of `Mustache.getHelper` should be optional.