alter p_get_first_heading() default $render value to true

Calls to p_get_first_heading() are most likely to expect to get the first heading even if it needs to be
generated, that is a $render value of true.  Only the metadata renderer itself needs a value of false in
order to prevent mutual dependency probelms.

This fix should finally complete FS#1010 & problems breadcrumbs have with use_first_headings when metadata
doesn't exist.  Also see, http://www.freelists.org/archives/dokuwiki/08-2007/msg00018.html &
http://www.freelists.org/archives/dokuwiki/08-2007/msg00132.html

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