The correction is similar to the other one for the other subclasses.
Also added tests for rowcount and rownumber during different fetch styles.
Just in case.
Regression introduced to fix ticket #80. Don't use fetchmany to get the
chunks of values. I did it that way because I was ending up into infinite
recursion calling __iter__ from __iter__: the solution has been the
"while 1: yield next()" idiom.
