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. 
@fugu13 is right for the oAuth case here. I don't think it practically matters that much in this case, but I don't think only one meaning of a word being implementable in one part of the spec should allow us to infer that meaning in another. So, I suggest we leave the "zero or more" language for basic auth.