Improve GsonEngine and ContentType registration

[mhagnumdw]

GsonEngine

• Allow to inherit from GsonEngine and define the creation of the Gson object
• Extract the Gson TypeAdapters that are inside the GsonEngine class
• The type adapter ISO8601DateTimeTypeAdapter can make use of com.google.gson.internal.bind.util.ISO8601Utils.parse(String, ParsePosition)
• GsonEngineTest is in the wrong place and is not even running by automatic tests
• Add tests for ISO8601
• The date parse for 2019-12-27T20:00:00.000Z does not work (the problem is this .000Z)
• Remove this type of thing (highlighted part): (date.getTime() / 1000) * 1000) - example here, here and here
• Update Gson from 2.3.1 to 2.8.6

ContentType

• The setTemplateEngine method, commonly called within Application.onInit(), must override the previously defined engine

[mhagnumdw]

Hi all!

What do you think of the above proposal? There is already a draft PR.

@decebals ,

The setTemplateEngine method, commonly called within Application.onInit(), must override the previously defined engine

If I have pippo-gson in pom.xml and register my JSON engine (which inherits from GsonEngine), it is not registered because Pippo previously registered GsonEngine.

I think we have to make some changes to ContentTypeEngines.registerContentTypeEngine. Do you suggest anything?

[mhagnumdw]

@decebals , when you have a little time, will it be if you can take a look here?

I don't know who else I can ping to take a look here.