fix custom drop special characters on validation

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[X] The commit message follows our guidelines: https://github.com/JsDaddy/ngx-mask/blob/develop/CONTRIBUTING.md#commit
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What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[X] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
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What is the current behavior?

when drop special characters is different from special characters the validation does not always work

example mask: +000 specialCharacters: ['+', ' '] dorpSpecialCharacters: [' '] value: +37

when the validation is done, it checks if dropSpecialCharacters is set, but when removing the characters from the value, it removed those from specialCharacters, and therefore the validation was not correct

JsDaddy / ngx-mask