Class field rename save error

What was the issue? 

When you create a class with a field named 'test' and then change the field name to 'Test' and save the class an error would appear.

How did you solve it? 

An additional check was added when checking if a key exists inside an array of existing fields.

Why is this fix important?

Now we have one less bug in Pimcore.

Are you planning on developing a new Pimcore feature? 

There are always some ideas for improving Pimcore.

And if you want to check out this pull request, just follow the link!