test cases for optional arguments in cdef functions

tests/errors/cdefoptargs.pyx

+def call5():
+    b(1,2,3,4,5)
+
+cdef b(a, b, c=1, d=2):
+    pass
+
+_ERRORS = """
+2:5:Call with wrong number of arguments (expected at most 4, got 5)
+"""

tests/run/cdefoptargs.pyx

+__doc__ = u"""
+    >>> call2()
+    >>> call3()
+    >>> call4()
+"""
+
+# the calls:
+
+def call2():
+    b(1,2)
+
+def call3():
+    b(1,2,3)
+
+def call4():
+    b(1,2,3,4)
+
+# the called function:
+
+cdef b(a, b, c=1, d=2):
+    pass