Add MessageSourceProperties

[ptahchiev]

As discussed on the gitter channel, if a user wants to create their own MessageSource bean, and use the properties from application.properties, he/she will not be able to inject those properties as an object, because those properties are fields on the MessageSourceAutoConfiguration class.

It would be nice to have a MessageSourceProperties object that holds the application properties of the MessageSource. However, as @dsyer explained, the the ApplicationContext is a MessageSource so that code gets executed super early and it was probably a defensive move to prevent too many things being initialized too early. It is well worth investigating how to overcome this issue.

[philwebb]

I missed the gitter conversation and I can't seem to find it now. I'm not totally following the description above, can you expand a little?

[wilkinsona]

We're binding directly to an auto-configuration class rather than to a dedicated configuration properties class. That's unusual and it'd be nice to remove the inconsistency. However, @dsyer wondered if the application context being a message source may make doing so a bit difficult.