Only perform expensive iteration though attribute names if passed-in map
didn't  already contain EVAL_SELECTION_TARGET_LOCAL_VARIABLE_NAME.
 Also removed unnecessary null checks.
Merge branch '2.1-master' of github.com:thymeleaf/thymeleaf into 2.1-master
Merge pull request #327 from jkuipers/2.1-master

Small WebVariablesMap optimization