tag_escape() should not strip digits

[sirzooro]

tag_escape() strips digits, but it shouldn't - they are needed for <h1>..<h6> tags. Attached patch fixes this.

[SergeyBiryukov]

tag_escape() is only used in gallery_shortcode(), so the bug manifests if someone sets itemtag or captiontag attribute to one of the <h1>..<h6> tags. This seems like a rare situation, but still possible.

[olleicua]

Tested using revision 18784M and above formatting.php.diff. It works.

[nacin]

Realistically, 1-6 is the only required range. I haven't seen any specs for introducing more header tags. Unless, I guess, they're allowed in namespaced elements. Looks good.

[ryan]

[18955]