Fix Affine-Cipher encrypt and drypt func

[shidian00]

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Describe your change:

• encrypt func should return alphabet instead of numbers string

• add checking for coprime as if coefficient 'a' is not coprime to 26 will reusultant in invalid result in inverse mod

• modify some test case

• Add an algorithm?

• Fix a bug or typo in an existing algorithm?

• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new JavaScript files are placed inside an existing directory.
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• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.