Fix Python negative unsigned default values

Closes #993

Examples/test-suite/default_args.i

@@ -156,6 +156,9 @@
       int double_if_handle_is_null(int n, MyHandle h = 0) { return h ? n : 2*n; }
       int double_if_dbl_ptr_is_null(int n, double* null_by_default)
         { return null_by_default ? n : 2*n; }
+
+      void defaulted1(unsigned offset = -1U) {} // minus unsigned!
+      void defaulted2(int offset = -1U) {} // minus unsigned!
   };
   int Foo::bar = 1;
   int Foo::spam = 2;

Examples/test-suite/python/default_args_runme.py

@@ -32,6 +32,8 @@ def run(module_name):
 
     f.newname()
     f.newname(1)
+    f.defaulted1()
+    f.defaulted2()
 
     if f.double_if_void_ptr_is_null(2, None) != 4:
         raise RuntimeError