Unwrapping boolean query consists of single MUST clause in parser

Solr query parser unwraps boolean queries with single SHOULD clause.

It works fine with OR query operator, but parser can’t unwrap single-clause query in {!q.op=AND} case.

I’m not familiar with edismax parser and this patch might affect it.
Single clause case looks pretty safe and existing tests were passed, but I ought to ask for peer review not only standard parser patch, but also it effect on edismax.