Application developer might want to include additional classes for as bootstrappers

This give them an easier way to do that without overriding the default options. E.g:

    protected function getBootstrappers()
    {
         return parent::getBootstrapper() + ['FooBoostrap']:
    }

Signed-off-by: crynobone <crynobone@gmail.com>
Only set the session if it's not null
Fixed a typo
Merge pull request #6348 from vlakoff/app-environment

[5.0] Handle Arrays In App::environment() And Add Tests
Merge pull request #6351 from GrahamCampbell/5.0-form

[5.0] Only Set The Session If It's Not Null
Merge pull request #6347 from crynobone/feature/configurable-bootstrapper

[5.0] Application developer might want to include additional classes for as bootstrappers
Rename method.
Shorten line.
Rename some traits to be less stupid.
Rename.
Fix a few imports.
Small rewrite.
Allow passing wildcards to App::environment().