Update spec
compare strings case-sensitively
If cursor is not on a helptag, return a blank string
Prevent bash from thinking fun() tags are bash functions

This raises a bash error:

    $ open http://vimdoc.sourceforge.net/htmldoc/eval.html#undotree()
    -bash: syntax error near unexpected token `('

Whereas this works fine:

    $ open "http://vimdoc.sourceforge.net/htmldoc/eval.html#undotree()"