[database:dump] Don't duplicate .gz file extension

[rpsu]

Currently passing both --file=[FILE] and --gz -options work fine. However, when using a [FILE] name ending in .gz extension the end result file name gets the .gz extension duplicated.

Would it make sense to not duplicate the file extension? I'm not sure if it breaks some automated workflows.
Currently
drupal database:dump --file=/vagrant/db_dumps/myfile.sql.gz --gz
results to
/vagrant/db_dumps/myfile.sql.gz.gz

I'm proposing it should result to
/vagrant/db_dumps/myfile.sql.gz

[rpsu]

This PR contains a check to avoid duplicated .gz file extensions. #3014

[jmolivas]

PR #3014 merged