Check if excluded key exists before deletion

[kelvinhammond]

Otherwise it throws an exceptions if the meta exclude include an extra key which is either already removed or doesn't exists such as 'id' or 'pk'.

[kvesteri]

I think it is desirable that the exclude returns exception for unknown keys. Imagine a situation where user makes a typo typing a field name in the exclude. I think we need to still think about this.

[kelvinhammond]

Superseded by #56