bug 2098: do not apply stemming if search term has digits

bug 2098: do not apply stemming if search term has digits

If a search contains a digit, it is almost certainly an identifier, and should
not be stemmed.  This is particularly relevant for ISBNs and ISSNs, which
can contain the letter "X" - for example, C4::Search::_build_stemmend_operand
would reduce "014100018X" to "x ", which for a MARC21 database would bring
up irrelevant results (e.g., "23 x 29 cm." from the 300$c).

With this patch, supplying a search term that contains one or more digits
followed by "X" will no longer retrieve irrelevant results.  This applies
to catalogs using Zebra and query stemming.

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