Contains the Last-Modified header returned by a GET method without accept headers.
Return None, if this live property is not supported.
Note that the last-modified date on a resource may reflect changes in any part of the state of the resource, not necessarily just a change to the response to the GET method. For example, a change in a property may cause the last-modified date to change. The getlastmodified property MUST be defined on any DAV compliant resource that returns the Last-Modified header in response to a GET.
This method SHOULD be implemented, especially by non-collections.