Support Properties subclasses in GsonTypes.getMapKeyAndValueTypes (#2758)

Author: panic08

gson/src/main/java/com/google/gson/internal/$Gson$Types.java

@@ -328,8 +328,8 @@ public static Type[] getMapKeyAndValueTypes(Type context, Class<?> contextRawTyp
      * class should extend Hashtable<String, String>, but it's declared to
      * extend Hashtable<Object, Object>.
      */
-    if (context == Properties.class) {
-      return new Type[] {String.class, String.class}; // TODO: test subclasses of Properties!
+    if (Properties.class.isAssignableFrom(contextRawType)) {
+      return new Type[] {String.class, String.class};
     }
 
     Type mapType = getSupertype(context, contextRawType, Map.class);

gson/src/test/java/com/google/gson/internal/GsonTypesTest.java

@@ -24,6 +24,7 @@
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.List;
+import java.util.Properties;
 import org.junit.Test;
 
 @SuppressWarnings("ClassNamedLikeTypeParameter") // for dummy classes A, B, ...
@@ -133,4 +134,17 @@ public <T> T method() {
       return null;
     }
   }
+
+  @Test
+  public void testGetMapKeyAndValueTypesForPropertiesSubclass() throws Exception {
+    class CustomProperties extends Properties {
+      private static final long serialVersionUID = 4112578634029874840L;
+    }
+
+    Type[] types =
+        $Gson$Types.getMapKeyAndValueTypes(CustomProperties.class, CustomProperties.class);
+
+    assertThat(types[0]).isEqualTo(String.class);
+    assertThat(types[1]).isEqualTo(String.class);
+  }
 }