Prioritize custom type conversion over standard conversions

[PaulSchweizer]

As discussed with @erikwrede we have to make sure that we prioritize registered custom type conversions over the defaults.

Example:

This:

from sqlalchemy_utils import JSONType

@convert_sqlalchemy_type.register(JSONType)
def convert_column_to_json(*args, **kwargs):
    return graphene.types.generic.GenericScalar

will result in a JSONString field instead of a GenericScalar

[adiberk]

This is pretty important, nice catch