RFC 3339 allows -00 and -00:00 offset

[thomasturrell]

The xAPI specification states:

A Timestamp MUST be formatted according to ISO 8601.
A Timestamp SHOULD* be expressed using the format described in RFC 3339, which is a profile of ISO 8601.

However the xAPI test suite reports the following errors if the timestamp is expressed using the format described in RFC 3339.

invalid date in timestamp: did not reject statement timestmap with -00 offset
invalid date in timestamp: did not reject statement timestmap with -00:00 offset
invalid date in timestamp: did not reject substatement timestamp with -00 offset
invalid date in timestamp: did not reject substatement timestamp with -00:00 offset

Section 4.3. 'Unknown Local Offset Convention' of RFC 3339 allows for negative zero offsets.