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. 
It isn't the LRS that uses a consumer secret -- the application isn't registered, so the LRS can't have used anything in regard to it. The LRS is just receiving a request. It's the AP that needs to use a blank consumer secret, and the LRS that needs to accept it (if it accepts this pattern). The idea that 'blank' in this context includes 'any number of spaces' is pretty bizarre, but whatever; I suspect the number that allow this mode of operation in practice will be vanishingly small.

Actually, I don't think this interpretation of blank is possible. The consumer secret isn't transmitted, so for the LRS to check if it was an unknown application would require it check the signature vs every possible length of spaces (infinitely many) as the consumer secret. The only viable interpretation of blank here is the empty string.