Simplify the object-to-list code in GsonRuntime#toList

It looks unnecessary to call #getAsJsonArray twice. It's probably just a `return this`, so not calling it is probably not really any performance improvement, but it makes the code easier to read.

jmespath-gson/src/main/java/io/burt/jmespath/gson/GsonRuntime.java

@@ -58,8 +58,9 @@ public List<JsonElement> toList(JsonElement value) {
     if (value.isJsonArray()) {
       return new JsonArrayListWrapper(value.getAsJsonArray());
     } else if (value.isJsonObject()) {
-      List<JsonElement> list = new ArrayList<>(value.getAsJsonObject().size());
-      for(Map.Entry<String, JsonElement> entry : value.getAsJsonObject().entrySet()) {
+      JsonObject object = value.getAsJsonObject();
+      List<JsonElement> list = new ArrayList<>(object.size());
+      for(Map.Entry<String, JsonElement> entry : object.entrySet()) {
         list.add(entry.getValue());
       }
       return list;