Define custom json encoder for qobj payloads

The qobj payload has a couple of quirks related to how qobj objects can
be constructed (and will be constructed after Qiskit/qiskit#3383
merges) that aren't handled by the default json encoder. The first is
that complex numbers are converted to the form "(real, imaginary)", and
the second is that numpy arrays are sometimes part of the payload.
Instead of relying on a layer outside of the provider coercing these
objects to the expected format this commit changes how we push qobj
payloads to the iqx api so that we run it through a json encoder that
understands how to do this. After this commit qiskit-ibmq-provider owns
it's own payload format and the generation of the expected json payload
format instead of relying on an external library to handle it for us.

Related to Qiskit#553

qiskit/providers/ibmq/api/rest/job.py

@@ -24,6 +24,7 @@
 from .validation import StatusResponseSchema
 from ..session import RetrySession
 from ..exceptions import ApiIBMQProtocolError
+from qiskit.providers.ibmq.utils import json_encoder
 
 
 class Job(RestAdapterBase):
@@ -137,7 +138,8 @@ def put_object_storage(self, url: str, qobj_dict: Dict[str, Any]) -> str:
         Returns:
             text response, that will be empty if the request was successful.
         """
-        response = self.session.put(url, json=qobj_dict, bare=True)
+        data = json.dumps(qobj_dict, cls=json_encoder.IQXJsonEconder)
+        response = self.session.put(url, data=data, bare=True)
         return response.text
 
     def get_object_storage(self, url: str) -> Dict[str, Any]:

qiskit/providers/ibmq/api/rest/root.py

@@ -21,6 +21,7 @@
 from .base import RestAdapterBase
 from .backend import Backend
 from .job import Job
+from qiskit.providers.ibmq.utils import json_encoder
 
 
 class Api(RestAdapterBase):
@@ -140,7 +141,8 @@ def job_submit(
         if job_tags:
             payload['tags'] = job_tags
 
-        return self.session.post(url, json=payload).json()
+        data = json.dumps(payload, cls=json_encoder.IQXJsonEconder)
+        return self.session.post(url, data=data).json()
 
     def submit_job_object_storage(
             self,

qiskit/providers/ibmq/utils/json_encoder.py

@@ -0,0 +1,33 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2020.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+# pylint: disable=method-hidden
+
+"""Custom JSON encoders."""
+
+import json
+from typing import Any
+
+
+class IQXJsonEconder(json.JSONEncoder):
+    """A json encoder for qobj"""
+
+    def default(self, o: Any):
+        # Convert numpy arrays:
+        if hasattr(o, 'tolist'):
+            return o.tolist()
+        # Use Qobj complex json format:
+        if isinstance(o, complex):
+            return (o.real, o.imag)
+        return json.JSONEncoder.default(self, o)