Removes the ambiguous word "blank" in 3 places.

Fix https://github.com/adlnet/xAPI-Spec/issues/471#issuecomment-67382428 as agreed on yesterdays call. 

Also addresses two other occurrences of the word "blank" in the spec. 

Also tidies up some lowercase shoulds around timestamp and authority. 
Well spotted on "The LRS" - I'll fix that in the next commit. 

Very interesting point about the consumer secret not being transmitted. If an empty string is the only viable interpretation of 'blank' here, then I'd argue that this interpretation also applies to line 2433 below.