Issue: case-sensitive date format

Dean Gaudet has pointed out to me that Section 3.3.1 needs to include
a sentence to the effect of

   HTTP-date is case sensitive and does not allow additional LWS
   beyond that specifically included as SP in the grammar.

since "literal" defaults to case-insensitive.

Just in case anyone wants to argue this point, I should mention that
all servers that I have tested with If-Modified-Since do treat the
value as case-sensitive, including Apache.

....Roy