Fixed top level extensions bug

[cumason123]

Fixes #64, #62, #32

Changes

import json
from cloudevents.sdk.http import CloudEvent, to_structured_http

attributes = {
    "type": "com.example.string",
    "source": "https://example.com/event-producer",
    "ext1": "testval"
}
data = json.dumps({"data-key": "val"})
event = CloudEvent(attributes, data)

headers, body = to_structured_http(event)
print(body)

Will now print

b'{"specversion": "1.0", "id": "45572bc9-4b9c-4230-9766-e4a5b180211d", "source": "https://example.com/event-producer", "type": "com.example.string", "time": "2020-07-22T07:23:17.117072+00:00", "data": "{\\"data-key\\": \\"val\\"}", "ext1": "testval"}'

Whereas before this change it would print non-top level extensions such as:

b'{"specversion": "1.0", "id": "be8e6b26-5a0a-47b7-8118-d9a6d84c181c", "source": "https://example.com/event-producer", "type": "com.example.string", "time": "2020-07-22T07:24:51.598809+00:00", "extensions": {"ext1": "testval"}, "data": "{\\"data-key\\": \\"val\\"}"}'

As you can see in the above, the attribute "extensions" should not be present in the marshalled json if I understand correctly. Furthermore, empty extensions dict's will no longer be marshalled as well.

One line description for the changelog

Top level extensions fix

• [*] Tests pass
• [*] Appropriate changes to README are included in PR

[grayside]

I have not read into the spec sufficiently to evaluate this requirement.