Fix the error message of KeyAlreadyPresent

Close #185

Author: frostming

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Fixed
 
+- Use the plain python string representation of `Key` in `KeyAlreadyPresent` error message. ([#185](https://github.com/sdispater/tomlkit/issues/185))
 - Fix the `astimezone()` and `replace()` methods of datetime objects. ([#188](https://github.com/sdispater/tomlkit/issues/188))
 - Add type definitions for `items()` function. ([#190](https://github.com/sdispater/tomlkit/issues/190))
 

tomlkit/exceptions.py

@@ -194,6 +194,7 @@ class KeyAlreadyPresent(TOMLKitError):
     """
 
     def __init__(self, key):
+        key = getattr(key, "key", key)
         message = f'Key "{key}" already exists.'
 
         super().__init__(message)