Fix PartialMethodObject (WIP)

Implemented feedback from PR #1522.
Does not pass new tests in test/completion/stdlib.py

test/completion/stdlib.py

@@ -169,12 +169,24 @@ X().a('')[0]
 #? str()
 X().a('')[1]
 
+#? int()
+X.a(X(), '')[0]
+#? str()
+X.a(X(), '')[1]
+
 tup = X().kw(1)
 #? int()
 tup[0]
 #? float()
 tup[1]
 
+tup = X.kw(X(), 1)
+#? int()
+tup[0]
+#? float()
+tup[1]
+
+
 def my_decorator(f):
     @functools.wraps(f)
     def wrapper(*args, **kwds):