Add config test

Author: adrienlauer

core/src/test/java/org/seedstack/seed/core/ConfigurationIT.java

@@ -187,6 +187,12 @@ public void json_expansion() {
         assertThat(configuration.get(String.class, "jsonValThroughSysProp.key2")).isEqualTo("val2");
     }
 
+    @Test
+    public void mapped_toString() {
+        Coffig configuration = injector.getInstance(Application.class).getConfiguration();
+        assertThat(configuration.toMappedString()).doesNotContain("${");
+    }
+
     @Test
     public void applicationInjection() {
         assertThat(application).isNotNull();