Allow to use Symfony 5 (#26)

Remove Symfony 4.2 deprecation notice (#25)

* Remove Symfony 4.2 deprecation notice

* Bad reference

Fix count error when no 'cc', 'bcc' or 'to' fields are set

Merge pull request #22 from geekdevs/feature/headers

Case-insensitive headers filtering

Merge pull request #21 from geekdevs/feature/headers

Added support for custom headers

Merge pull request #20 from geekdevs/feature/plugins

Added support for SwiftMailer plugins

Fix personalization object

Fix minimum requirement and add Test

Drop php 5 support

Update README