Unwrap errors from value field if present

The W3C spec says that all responses from the remote end
should be set as a `value` field. I think most people
thought that this didn't apply to errors, but they would
think wrong. It does. *sigh*

Geckodriver 0.15 follows the W3C spec more closely, and
so correctly wraps the error in the `value` property of
the returned JSON. Handle this nicely.

Author: shs96c

java/client/src/org/openqa/selenium/remote/http/W3CHttpResponseCodec.java

@@ -22,6 +22,7 @@
 import static java.net.HttpURLConnection.HTTP_OK;
 
 import com.google.common.base.Strings;
+import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
 import com.google.gson.JsonParser;
 
@@ -78,6 +79,11 @@ public Response decode(HttpResponse encodedResponse) {
       log.fine("Processing an error");
       JsonObject obj = new JsonParser().parse(content).getAsJsonObject();
 
+      JsonElement w3cWrappedValue = obj.get("value");
+      if (w3cWrappedValue instanceof JsonObject && ((JsonObject) w3cWrappedValue).has("error")) {
+        obj = (JsonObject) w3cWrappedValue;
+      }
+
       String message = "An unknown error has occurred";
       if (obj.has("message")) {
         message = obj.get("message").getAsString();