MINOR: compression: Improve the way Vary header is added

MINOR: compression: Improve the way Vary header is added

When a message is compressed, A "Vary" header is added with
"accept-encoding" value. However, a new header is always added, regardless
there is already a Vary header or not. In addition, if there is already a
Vary header, there is no check on values to be sure "accept-encoding" value
is not already there. So it is possible to have it twice.

To improve this part, we now test Vary header values and "accept-encoding"
is only added if it was not found. In addition, "accept-encoding" value is
appended to the last Vary header found, if any. Otherwise, a new header is
added.