added test cases

Author: codewithmide

Caesars_Cipher/Caesars_Cipher.md

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+One of the simplest and most widely known ciphers is a `Caesar cipher`, also known as a `shift cipher`. In a shift cipher the meanings of the letters are shifted by some set amount.
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+A common modern use is the `[ROT13](https://www.freecodecamp.org/news/how-to-code-the-caesar-cipher-an-introduction-to-basic-encryption-3bf77b4e19f7/) cipher`, where the values of the letters are shifted by 13 places. Thus `A ↔ N`, `B ↔ O` and so on.
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+Write a function which takes a `[ROT13](https://www.freecodecamp.org/news/how-to-code-the-caesar-cipher-an-introduction-to-basic-encryption-3bf77b4e19f7/) cipher` encoded string as input and returns a decoded string.
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+All letters will be uppercase. Do not transform any non-alphabetic character (i.e. spaces, punctuation), but do pass them on.