Demonstrate a regression when C<> is used in a =headN block
Fix regression with C<> formatting codes inside =headN sections

With 29306b7, headers stash unformatted text in $_[0]{htext} for use in
the TOC and generating section IDs.  Code sections (C<>) were handled
separately inside handle_text and never appended to $_[0]{htext},
meaning headers lacked those pieces of text.  Adjust handle_text to
always append to the htext stash if inside a header.

The regression tests in t/xhtml10.t now pass.

Resolves [rt.cpan.org #82706].
Merge pull request #46 from tsibley/code-in-head-regression

Fix code (C<>) in head regression