test for some problematic **kw use cases

tests/run/kwargproblems.pyx

+__doc__ = """
+    >>> d = {1 : 2}
+    >>> test(**d)
+    Traceback (most recent call last):
+    TypeError: test() keywords must be strings
+    >>> d
+    {1: 2}
+
+    >>> d = {}
+    >>> test(**d)
+    {'arg': 3}
+    >>> d
+    {}
+
+    >>> d = {'arg' : 2}
+    >>> test(**d)
+    {'arg': 3}
+    >>> d
+    {'arg': 2}
+"""
+
+def test(**kw):
+    kw['arg'] = 3
+    return kw