[#22] Support string type in stack parameter converter

Set type of stack parameter to StringType in type-switch statement of
toStackParameter function.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/morph/client/client_test.go

@@ -20,6 +20,10 @@ func TestToStackParameter(t *testing.T) {
 			value:   int64(100),
 			expType: sc.IntegerType,
 		},
+		{
+			value:   "hello world",
+			expType: sc.StringType,
+		},
 	}
 
 	for _, item := range items {