Fix issue where pytest.raises() doesn't always return Exception instance in py26

Fixes #767

_pytest/python.py

@@ -1099,6 +1099,13 @@ class RaisesContext(object):
         __tracebackhide__ = True
         if tp[0] is None:
             pytest.fail("DID NOT RAISE")
+        if sys.version_info < (2, 7):
+            # py26: on __exit__() exc_value often does not contain the
+            # exception value.
+            # http://bugs.python.org/issue7853
+            if not isinstance(tp[1], BaseException):
+                exc_type, value, traceback = tp
+                tp = exc_type, exc_type(value), traceback
         self.excinfo.__init__(tp)
         return issubclass(self.excinfo.type, self.ExpectedException)