fix: body encoding for rest transport (#768)

Basically just replace `json` argument with `data`

Apparently, the `json` parameter in requests.Session.request() method does not expect JSON string,
but expects python dictionary instead, which is not intuitive and does not even match the
documentation of the method: https://github.com/psf/requests/blob/master/requests/sessions.py#L483.

At the same time in the Quickstart, it is explicitly said that `json` parameter was added in
version `2.4.2` and expects python `dict`, while `data` argument can process raw encoded json
string.

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest.py.j2

@@ -198,7 +198,7 @@ class {{ service.name }}RestTransport({{ service.name }}Transport):
         {% if not method.void %}response = {% endif %}self._session.{{ method.http_opt['verb'] }}(
             url
             {%- if 'body' in method.http_opt %},
-            json=body,
+            data=body,
             {%- endif %}
         )