Because the category makes the protocol apply to all `NSMutableDictionaries`, it can't assume keys are `NSString`. Because these types are covariant, the change is not breaking.

TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods

The GRPCCall import must be in the header because it defines a protocol
that GRPCRequestHeaders uses.

The GRPCCall.h file is already correctly imported in the
GRPCRequestHeaders.m implementation file. This header shouldn’t need to
import GRPCCall and should specify a class forward declaration for
reference.

So we can make the property a dictionary later, by just extending
NSMutableDictionary to conform to the protocol.