Disable set.update() optimization due to Python incompatibility.

set.update() is actually variadic.  Fixes #1645.

Cython/Compiler/Builtin.py

@@ -319,8 +319,9 @@ builtin_types_table = [
                                     BuiltinMethod("clear",   "T",  "r", "PySet_Clear"),
                                     # discard() and remove() have a special treatment for unhashable values
 #                                    BuiltinMethod("discard", "TO", "r", "PySet_Discard"),
-                                    BuiltinMethod("update",     "TO", "r", "__Pyx_PySet_Update",
-                                                  utility_code=UtilityCode.load_cached("PySet_Update", "Builtins.c")),
+                                    # update is actually variadic (see Github issue #1645)
+#                                    BuiltinMethod("update",     "TO", "r", "__Pyx_PySet_Update",
+#                                                  utility_code=UtilityCode.load_cached("PySet_Update", "Builtins.c")),
                                     BuiltinMethod("add",     "TO", "r", "PySet_Add"),
                                     BuiltinMethod("pop",     "T",  "O", "PySet_Pop")]),
     ("frozenset", "PyFrozenSet_Type", []),

tests/run/set.pyx

@@ -88,6 +88,18 @@ def test_set_update(v=None):
     return s1
 
 
+def test_set_multi_update(a, b, c):
+    """
+    >>> type(test_set_multi_update()) is set
+    True
+    >>> sorted(test_set_multi_update())
+    """
+    ['a', 'b', 'c', 1, 2, (1, 2)]
+    cdef set s1 = set()
+    s1.update('abc', set([1]), frozenset((1,2)))
+    return s1
+
+
 def test_object_update(v=None):
     """
     >>> type(test_object_update()) is set