This is done by treating any object with __iter__ methods the way only
'list' and 'tuple' objects were treated before. Except dictionaries,
they have an __iter__ method too, but that's not what we want.

All 291 tests aswell as the new one pass.

The two test cases test, respectively, (1) context precedence and
(2) that section output not be rendered.  The two test cases were
originally proposed for inclusion in the Mustache spec test cases
in mustache/spec issues #31 and #32:

* https://github.com/mustache/spec/pull/31
* https://github.com/mustache/spec/pull/32