Expected Behavior

from cloudevents.http import from_http
headers = {
    "Ce-Source": "<my-source>",
    "Ce-Specversion": "1.0",
    "Ce-Type": "my-type",
    "Ce-Id": str(uuid4())
}
from_http("", headers)

should pass despite upper-cased attributes names.

from cloudevents.http import from_http
headers = {
    "ce-source": "<my-source>",
    "ce-specversion": "1.0",
    "ce-type": "my-type",
    "ce-id": str(uuid4())
}
from_http(None, headers)

likewise, this should return an event with data set to none.

Actual Behavior

None data causes _json_or_string to fail, Recommend addding a None if statement in that function.

Capitalized ce-specversion isn't read by from_http. Instead, from_http should make all keys lower case before passing it into future functionality.