[SQLObject] orderBy isinstance str check

Hi,

I recently discovered that some of my code was passing unicode strings to
orderBy and that this worked as long as the strings didn't start with "-".
When they did start with "-", the _mungeOrderBy check (line 69 in
sresults.py) didn't detect the "-" because the strings were of type unicode
and not type str and _mungeOrderBy checks for str and not basestring.

I was just wondering if this was on purpose or not as it looks like there
are other places in the sqlobject code that do check for type basestring.

Andrew