ISSUE-104: Value of progress event total when it cannot be determined

ISSUE-104: Value of progress event total when it cannot be determined

http://www.w3.org/2005/06/tracker/webapi/issues/104

Raised by: Charles Mccathienevile
On product: Progress Event

What should this be. I don't think we should set a very big number and 
hope that nobody needs to deal with that. I am more inclined to set it 
at zero. If you get zero, either the thing is empty or undefined - 
still not ideal.

If you leave it as undefined, then people will have to check 
lengthComputable before they know if the value is meaningful.

The alternative is to make it a negative value but I am not mad keen on 
doing that.