message-types: support obj-path and signature

Change-Id: I7cb1563af3375f2a8b22c78ac59f7e69f176933f
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/test/message/types.cpp b/test/message/types.cpp
index 4172cb6..8676e25 100644
--- a/test/message/types.cpp
+++ b/test/message/types.cpp
@@ -29,5 +29,12 @@
 
         assert(dbus_string(a, std::move(b), c) == "sss");
     }
+
+    // Check object_path and signatures.
+    assert(dbus_string(std::string("asdf"),
+                       sdbusplus::message::object_path("/asdf"),
+                       sdbusplus::message::signature("sss")) ==
+           "sog");
+
     return 0;
 }