Error serializing field duration = models.DurationField()?

Start by running the supplied Asset model, AssetNode schema, and duration query against the listed environment to reproduce the error. Inspect graphql/type/scalars.py around line 51 and GraphQLScalarType around line 65, then verify that querying a non-null Django DurationField completes without the reported datetime.timedelta serialization error.

Hi there, i'm not 100% sure if this is a bug or not (I'm newbee here) but i got a problem while querying a Node that uses the field type models.DurationField ( it really returns datetime.timedelta).

Traceback (most recent call last):
  File "/tmp/leaky_venv/lib/python3.6/site-packages/promise/promise.py", line 65, in try_catch
    return (handler(*args, **kwargs), None)
  File "/tmp/leaky_venv/lib/python3.6/site-packages/graphql/execution/executor.py", line 375, in <lambda>
    resolved
  File "/tmp/leaky_venv/lib/python3.6/site-packages/graphql/execution/executor.py", line 399, in complete_value
    return complete_leaf_value(return_type, result)
  File "/tmp/leaky_venv/lib/python3.6/site-packages/graphql/execution/executor.py", line 440, in complete_leaf_value
    return return_type.serialize(result)
  File "/tmp/leaky_venv/lib/python3.6/site-packages/graphql/type/scalars.py", line 51, in coerce_float
    return float(value)
TypeError: float() argument must be a string or a number, not 'datetime.timedelta'

This error points to line 51 but i think the class GraphQLScalarType (on line 65) is also related.

Here is my env's prop

PostgreSQL 10.0 on x86_64-pc-linux-gnu (xubuntu 16.04), compiled by gcc (Debian 6.3.0-18) 6.3.0 20170516, 64-bit

Python == 3.6.1

Django==1.11.6
django-filter==1.1.0
graphene==2.0
graphene-django==2.0.0
graphql-core==2.0
graphql-relay==0.4.5

How to reproduce?

1- Create your model

class Asset(models.Model):
    duration = models.DurationField(null=True, blank=True, help_text="Duration of a given offer/product (auto calculated)")
   
    class Meta:
        ordering = ["id"]

2-Create your schema

class AssetNode(DjangoObjectType):

    class Meta:
        model = Asset
        interfaces = (relay.Node, )

3- insert a not-null value on the duration field... and run your query.

{assets(first:20,) {
  edges {
    node {
      id
      duration
    }
  }
}}

and you should que the error:

{
      "message": "float() argument must be a string or a number, not 'datetime.timedelta'"
    }