Fix calling the unicode object using keyword args

before it produced the following exception:
TypeError: unicode() got multiple values for keyword argument 'encoding'

src/runtime/types.cpp

@@ -813,7 +813,7 @@ static Box* typeCallInner(CallRewriteArgs* rewrite_args, ArgPassSpec argspec, Bo
 
         ParamReceiveSpec paramspec(4, 3, false, false);
         bool rewrite_success = false;
-        static ParamNames param_names({ "string", "encoding", "errors" }, "", "");
+        static ParamNames param_names({ "", "string", "encoding", "errors" }, "", "");
         static Box* defaults[3] = { NULL, NULL, NULL };
         Box* oargs[1];
 

test/tests/unicode_test.py

@@ -165,3 +165,4 @@ class C(object):
         return MyUnicode("hello world")
 
 print type(unicode(C()))
+print unicode("test", encoding="UTF-8")