MBAPIHelper: introduce _make_inc_arg() to build inc parameter value

- it produces a more stable output (elements are sorted)
- it gets rid of empty elements (preventing a++b case)
- it accepts any iterable
- add a test for it

I also removed 2 useless conversion to tuple() (it can use a set() anyway)

test/test_api_helpers.py

@@ -98,7 +98,7 @@ class MBAPITest(PicardTestCase):
         self.assertEqual(value, unencoded_query_args[argname])
 
     def _test_inc_args(self, ws_function, arg_list):
-        self.assertInQuery(self.ws.get_url, 'inc', "+".join(arg_list))
+        self.assertInQuery(self.ws.get_url, 'inc', self.api._make_inc_arg(arg_list))
 
     def test_get_release(self):
         inc_args_list = ['test']
@@ -212,6 +212,11 @@ class MBAPITest(PicardTestCase):
         with self.assertRaises(StopIteration):
             next(generator)
 
+    def test_make_inc_arg(self):
+        result = self.api._make_inc_arg(['b', 'a', '', 1, (), 0])
+        expected = '1+a+b'
+        self.assertEqual(result, expected)
+
 
 class AcoustdIdAPITest(PicardTestCase):