[Internet::Password] Improve mix_case and special_characters support

[meuble]

Ensure password gets at least 1 letter when mix_case and special_characters are both required and doing so even on really small length.

Issue

When activating both mix_case and special_characters flags, it was possible to result in a password without any letter.

Example :

irb(main):044:0> Faker::Internet.password(min_length: 4, max_length: 6, mix_case: true, special_characters: true)
=> "&&%$9"
irb(main):045:0> Faker::Internet.password(min_length: 48, max_length: 48, mix_case: true, special_characters: true)
=> "^##&!@&%$%**@!\#@#\#@#^!@@&$!!^*!$*&^%!&!#%&*!#%&!"

This is due to the special characters blocs replacing between 1 and min_length characters, which might be bigger than the number of letters in the string (which is a minimum of 2 when mix_case is enabled). With bad luck, all letters could end up being all replaced.

This issue can raise errors when testing the uniqueness of passwords with case sensitivity.

[farhatahmad]

I ran into this issue today and I'm surprised this isn't a bigger issue. I would expect mix_case to ensure the presence of both a lower case and an uppercase character when generating a password, but that's definitely not the case.

This seems like an improvement over the current logic

[meuble]

I forgot about this PR since then, thanks @farhatahmad for the reminder ^_^

Can anyone have a look ? @vbrazo maybe ?

[meuble]

Up ?

[antonioeloi]

This should be added to master as soon as possible as it's a good improvement over the internet:password generation. Thank you @meuble.