MEDIUM: compression: consider the "q=" attribute in Accept-Encoding

MEDIUM: compression: consider the "q=" attribute in Accept-Encoding

Till now we didn't consider "q=". It's problematic because the first
effect is that compression tokens were not even matched if it was
present.

It is important to parse it correctly because we still want to allow
a user-agent to send "q=0" to explicitly disable a compressor, or to
specify its preferences.

Now, q-values are respected in order of precedence, and when several
q-values are equal, the first occurrence is used.