MEDIUM: filters: implicitly enable "decomp-req" and "decomp-res" when decompression...

MEDIUM: filters: implicitly enable "decomp-req" and "decomp-res" when decompression options are used

If the "decomp-req" and/or "decomp-res" filters are not declared
explicitly but decompression options are set for them, then we
detect that the options relevant for one or the other were set and
we enable the filter automatically. This eases the configuration for
users who forget to declare the relevant decompression filters, as
we already do for compression ones.

However, when multiple filters are involved on a single proxy, it is
highly recommended to declare filters explicitly in order to avoid
surprises and this way it is easier to control in which order they
are executed. See also "filter-sequence" to help control executing
order.