TAXII Version Filter - Please review

[Bret Jordan <Bret_Jordan@symantec.com>]

All,

Drew, Allan, and I took a stab at fixing the text on the version filter parameter so that can work with a STIX object that does not have a modified timestamp (marking-definition object).  The text from section 3.4.1 now reads:

### BEGIN

For STIX objects, this filter option requests objects whose modified time matches exactly the provided value and the value MUST follow the rules for timestamp as defined in [STIX™ Version 2.0. Part 1: STIX Core Concepts]. For STIX objects that do not contain a modified timestamp (ex. the marking-definition object), then this filter should match on the created timestamp. If the STIX object does not contain either a modified or created timestamp, then this filter should return the latest version according to the server.

For example: "2016-01-01T01:01:01.000Z" tells the server to return the exact STIX object(s) that matched the modified time or created time (in the case of a marking-definition object) of "2016-01-01T01:01:01.000Z".

### END

The extra caveat for not having either a modified or created timestamp deals with the potential cyber observable changes. Please review and comment on the list with any concerns or changes. 

Bret