fix multiple encoding in nested templates

okay i've made some little tests regarding my comment at bcosca/fatfree#656.
It looks like we can just skip the whole encoding for sub-templates, since the hive that had already been encoded is passed into the sub-templates (<include> tag uses get_defined_vars(); ).
Maybe you can check that too. If you merge it, it'll also be included in the PR to bcosca (i think so).