Fix handling of negated fulltext keys

As documented in the code, we currently handle the settings in fulltext keys arrays like this:

#conjunction | #negation | return value
----------------------------------------------------------------
AND          | FALSE     | A B C
AND          | TRUE      | -(A B C)
OR           | FALSE     | ((A) OR (B) OR (C))
OR           | TRUE      | -A -B -C

However, I now found out that the queries for both negated cases (-(A B C) and -A -B -C) are actually equivalent, doing what I'd suppose of the latter. It seems that in a negated sub-expression Solr automatically switches the default operator – rather clever, but in this case not what we want.