Backport 4.1 Application::register() to also run boot() when app is already booted.

Backport 4.1 Application::register() which would also run the service provider boot() method if the application already booted. This would ease the requirement to manually boot service provider based on environment.

> **Pre-caution**: Might cause merge-conflict when merging 4.0 to master if this is merged to 4.0.

Signed-off-by: crynobone <crynobone@gmail.com>