Allow the BeanToJsonConverter cleanly handle common return types

The way that `toJson` is currently handled encourages people
to take a hard dependency on gson, or attempt to handle
serialising JSON themselves. Modify how `toJson` is handled
to allow us to return other data types, such as `Map` and
anything extending `Collection`.

This opens up the possibility of an infinite loop. Best not
do that in the Real World, eh?

java/client/src/org/openqa/selenium/remote/BeanToJsonConverter.java

@@ -198,10 +198,17 @@ private JsonElement convertObject(Object toConvert, int maxDepth) throws Excepti
         if (res instanceof JsonElement) {
           return (JsonElement) res;
         }
-        try {
-          return new JsonParser().parse((String) res);
-        } catch (JsonParseException e) {
-          return new JsonPrimitive((String) res);
+
+        if (res instanceof Map) {
+          return convertObject(res);
+        } else if (res instanceof Collection) {
+          return convertObject(res);
+        } else if (res instanceof String) {
+          try {
+            return new JsonParser().parse((String) res);
+          } catch (JsonParseException e) {
+            return new JsonPrimitive((String) res);
+          }
         }
       } catch (ReflectiveOperationException e) {
         throw new WebDriverException(e);

java/client/test/org/openqa/selenium/remote/BeanToJsonConverterTest.java

@@ -27,12 +27,14 @@
 import static org.junit.Assert.fail;
 
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSortedSet;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 import com.google.gson.Gson;
 import com.google.gson.JsonArray;
 import com.google.gson.JsonObject;
 import com.google.gson.JsonParser;
+import com.google.gson.JsonPrimitive;
 import com.google.gson.JsonSyntaxException;
 
 import org.junit.Test;
@@ -233,6 +235,39 @@ public void testShouldCallToJsonMethodIfPresent() {
     assertEquals("\"converted\"", json);
   }
 
+  @Test
+  public void toJsonMethodCanConvertibleReturnedMap() {
+    class ToJsonReturnsMap {
+      public Map<String, Object> toJson() {
+        return ImmutableMap.of("cheese", "peas");
+      }
+    }
+
+    String json = new BeanToJsonConverter().convert(new ToJsonReturnsMap());
+    JsonObject converted = new JsonParser().parse(json).getAsJsonObject();
+
+    assertEquals(1, converted.entrySet().size());
+    assertEquals("peas", converted.get("cheese").getAsString());
+  }
+
+  @Test
+  public void toJsonMethodCanConvertReturnedCollection() {
+    class ToJsonReturnsCollection {
+      public Set<String> toJson() {
+        return ImmutableSortedSet.of("cheese", "peas");
+      }
+    }
+
+    String json = new BeanToJsonConverter().convert(new ToJsonReturnsCollection());
+    JsonArray converted = new JsonParser().parse(json).getAsJsonArray();
+
+    assertEquals(2, converted.size());
+    JsonArray expected = new JsonArray();
+    expected.add(new JsonPrimitive("cheese"));
+    expected.add(new JsonPrimitive("peas"));
+    assertEquals(expected, converted);
+  }
+
   @Test
   public void testShouldCallAsMapMethodIfPresent() {
     String json = new BeanToJsonConverter().convert(new Mappable1("a key", "a value"));