Change HTTPBody length stored integer from Int to Int64

[czechboy0]

Change HTTPBody length stored integer from Int to Int64.

This will be an API break, so must happen as part of a minor before 1.0 or as part of 1.0 itself.

[simonjbeaumont]

This will be an API break, so must happen as part of a minor before 1.0 or as part of 1.0 itself.

To help roll this out, we could rename this type at the same time to contentLength and deprecate length?

[czechboy0]

Yeah that's a good idea, that way we can land now and clean up the deprecated one for 1.0.