Log a warning when a @RootContext field isn't populated

When a bean is injected in another class which has a different Context
from the required @RootContext field class, the field is left null. This
could produce hard to find bugs. The fix adds a log to warn the user.
Merge pull request #678 from tbruyelle/674_WarningOnNullRootContext

Log a warning when a @RootContext field isn't populated