Allow using a custom resolver function on DjangoConnectionField

[mb1511]

We ran into an issue the other day trying to add a custom resolver function to a DjangoConnectionField:

class Query(ObjectType)
    some_field = DjangoConnectionField(SomeType, resolver=some_resolver)

In this case the some_resolver function is simply ignored.

We managed to work around it by defining the resolver on the query using the default def resolve_{field}: pattern and just linked that to return the result of the function we wanted to use.

Admittedly, its a pretty niche use case, but still thought it would be nice to try and fix.