Allowing for camel and studly-cased mutator attributes

In [this commit](https://github.com/laravel/framework/commit/e5c3c1cb36021e351bee2b56f2bcbccacfa494eb#diff-d8e24a5815a26f7211e66db787be647f) a previously undocumented feature of the mutator attributes was removed. Prior to this commit, it was possible to reference accessors using snake, camel, and studly cases. For example:

```php
protected $appends = ['first_name', 'lastName', 'FullName'];

public function getFirstNameAttribute() {...}
public function getLastNameAttribute() {...}
public function getFullNameAttribute() {...}
```

This would work and append the key that you specified into the resulting array:

```php
[
    'first_name' => ...,
    'lastName' => ...,
    'FullName' => ...,
]
```

After this commit, it only accepts snake case. I imagine we aren't the only ones using this (and we are using it rather widely in our systems), so I put together a pull request that keeps all the current features and allows for this. I added a test to verify that it works as well. Let me know if you need anything else or if you have any comments/questions/concerns.
@GrahamCampbell sorry, I'll try to avoid this in the future :)