[PATCH] Reset single-row processing mode at end of pipeline commands queue

Hello,

I'm trying to make single-row mode and pipeline mode work together in 
Psycopg using libpq. I think there is something wrong with respect to 
the single-row mode flag, not being correctly reset, in some situations.

The minimal case I'm considering is (in a pipeline):
* send query 1,
* get its results in single-row mode,
* send query 2,
* get its results *not* in single-row mode.

It seems that, as the command queue in the pipeline is empty after 
getting the results of query 1, the single-row mode flag is not reset 
and is still active for query 2, thus leading to an unexpected 
PGRES_SINGLE_TUPLE status.

The attached patch demonstrates this in the test suite. It also suggests 
to move the statement resetting single-row mode up in 
pqPipelineProcessQueue(), before exiting the function when the command 
queue is empty in particular.

Thanks for considering,
Denis