fix: Faker::Internet.username should not generate duplicated punctuation

[thdaraujo]

[fixes #2967]

In #2950 there was a change on how email usernames were generated. This caused usernames to be generated with duplicate punctuation - for example:

Faker::Internet.email(name: 'Mr. Faker')
#=> faker.mr.@test.example
#=> mr._faker@test.example  

This PR restores the previous behavior when dealing with punctuation.

While we can't guarantee that any of these generated emails will be valid, we can restore the previous behaviour.