MessageSourceAutoConfiguration should only check the current context for a message source bean

[dsyer]

The @Conditional on MessageSourceAutoConfiguration checks for an existing bean of type MessageSource so a parent context can switch it off, even if the parent is empty (the default). We should switch to only check the current context.

[wilkinsona]

This got fixed in 1.0.x and merged straight into master, i.e. the change isn't in 1.2.x