evergreen.maintain_901() can mangle its output

If a MARCXML record has some elements that use a namespace prefix and some do that do not, the regexp used to insert the 901 can insert it in the wrong place.

For example, consider:

<marc:record xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.loc.gov/MARC21/slim" xmlns:marc="http://www.loc.gov/MARC21/slim" xsi:schemaLocation="http://www.loc.gov/MARC21/slim http://www.loc.gov/ standards/marcxml/schema/MARC21slim.xsd">
<leader>01234cam a22003014a 4500</leader>
<controlfield tag="001">216938607</controlfield>
<controlfield tag="003">OCoLC</controlfield>
<datafield tag="521" ind1=" " ind2=" ">
  <subfield code="a">RL: 3 ; ages 7-12.</subfield>
</datafield>
<datafield tag="800" ind1="1" ind2=" ">
  <subfield code="a">Warner, Gertrude Chandler,</subfield>
  <subfield code="d">1890-1979.</subfield>
  <subfield code="t">Boxcar children mysteries.</subfield>
</datafield>
</marc:record>

maintain_901() would munge the record to something like this:

<marc:record xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.loc.gov/MARC21/slim" xmlns:marc="http://www.loc.gov/MARC21/slim" xsi:schemaLocation="http://www.loc.gov/MARC21/slim http://www.loc.gov/ standards/marcxml/schema/MARC21slim.xsd">
<leader>01234cam a22003014a 4500</leader>
<controlfield tag="001">216938607</controlfield>
<controlfield tag="003">OCoLC</controlfield>
<datafield tag="521" ind1=" " ind2=" ">
  <subfield code="a">RL: 3 ; ages 7-12.<datafield tag="901" ind1=" " ind2=" "><subfield code="a">216938607</subfield><subfield code="b">OCoLC</subfield><subfield code="c">5322176</subfield><subfield code="t">biblio</subfield></datafield></subfield>
</datafield>
<datafield tag="800" ind1="1" ind2=" ">
  <subfield code="a">Warner, Gertrude Chandler,</subfield>
  <subfield code="d">1890-1979.</subfield>
  <subfield code="t">Boxcar children mysteries.</subfield>
</datafield>
</marc:record>

That's not a valid MARCXML record. If one is attempting to reingest that record to add the 901 field (see bug 798702 for why one would do that), you can end up with a misleading error message. In particular, running

update biblio.record_entry set id = id where id = 5322176;

for such a record can get you an error like this:

psql:x.sql:2: ERROR: Tag "" is not a valid tag. at /usr/share/perl5/MARC/File/SAX.pm line 92
CONTEXT: PL/Perl function "maintain_control_numbers"

Evergreen: master