bug 2235: fix various data-corrupting bugs in MARC editor

bug 2235: fix various data-corrupting bugs in MARC editor

1. Non-ASCII characters in the 942$h (biblioitems.cn_class)
   would cause an edit to fail and lead to a corrupt MARC record in
   biblioitems.marc and biblioitems.marcxml.  Fixed by
   turning on Perl's UTF-8 flag for MARC tag parameters
   processed by C4::Biblio::TransformHtmlToMarc

2. Do not add empty 006 and 007 fields if the corresponding
   input fields are blank.

3. Make it possible to save '0' as a valid subfield
   value.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>