added HTTP HEAD section

Resolves issue: https://github.com/adlnet/xAPI-Spec/issues/72

Note: in the issue we agreed that if "Content-Length header may be ignored and that if a value is present, it can't be relied upon as having any significance", however since the HTTP spec allows this header to be omitted, but not for the 'wrong' value to be return I've instead just noted that it may be omitted.