Fix Dewey call number sorting

Fix Dewey call number sorting

Ported over Koha commit aef8358c - fix for Koha Bug 4265.
Further documented in Evergreen LP # 902667
Here's the description from the commit message by Magnus Enger:

C4::ClassSortRoutine::Dewey turns "306 Les" into "306_Les"
for items.cn_sort and MARC-field 952$6, which results in
"306.46 Les" being sorted before "306 Les" in the OPAC.
With this patch, "306 Les" is turned into "306_000000000000000_Les".

Signed-off-by: James Fournie <jfournie@sitka.bclibraries.ca>
Signed-off-by: Dan Scott <dscott@laurentian.ca>