Using line breaking and <hr /> results in an invalid markup

Line breaking filter does not treat hr as a block level element and encloses it in paragraph tags (if used with empty lines above and below). This is invalid in XHTML.

Attached patch adds the hr tag to a list of block elements in _filter_autop. Possibly some more tags should be added (fieldset?, center for html?), but the list doesn't correspond to any DTD, so it's hard to tell.

  $block = '(?:table|thead|tfoot|caption|colgroup|tbody|tr|td|th|div|dl|dd|dt|ul|ol|li|pre|select|form|blockquote|address|p|h[1-6]|hr)';

Patch is created against the head, but tested only on the 5.6 branch (code is the same).