Explicitly use simple when processing ticket Description search strings

It ensures consistency with the FUNCTION parameter which already uses
'simple' configuration for to_tsvector(), which is also consistent with the
following index:

    CREATE INDEX Tickets7 ON Tickets USING GIN(to_tsvector('simple', Description));

This fixes a test failure in t/ticket/search.t: searching "database"
could be interpreted as the its stem "databas" (depending on
default_text_search_config) instead of "database" the "simple" config
returns.

Author: sunnavy

lib/RT/Tickets.pm

@@ -819,7 +819,7 @@ sub _FullTextStringLimit {
             %rest,
             FUNCTION    => "to_tsvector('simple', main.$field)",
             OPERATOR    => '@@',
-            VALUE       => 'plainto_tsquery(' . $dbh->quote($value) . ')',
+            VALUE       => q{plainto_tsquery('simple', } . $dbh->quote($value) . ')',
             QUOTEVALUE  => 0,
         );
     }