Configuration option to disable HTML escaping when using Gson (#298)

* Configuration option to disable HTML escaping when using Gson

The default implementation of Gson will escape certain characters by default. This
includes the `=` character, which is used in base64 encoding and cause problems when
deserializing the value to a base64 encoded string in a service.

Adding an option for disabling this feature makes it easier to generate client code
with sane defaults.

* Update Petstore sample

samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/JSON.java

@@ -64,7 +64,8 @@ public class JSON {
             }
           })
         ;
-        return fireBuilder.createGsonBuilder();
+        GsonBuilder builder = fireBuilder.createGsonBuilder();
+        return builder;
     }
 
     private static String getDiscriminatorValue(JsonElement readElement, String discriminatorField) {