netty#15220)

…217)

Motivation:
Right now the default compression level inside the used Deflater is
defined as "-1", which according to zlib manual also defaults to "6"
which is used inside netty as the default. That said, we should also
allow to provide Deflater.DEFAULT_COMPRESSION (-1) as an argument.

Modifications:
The range check has been modified to allow for -1 and it now uses the
constants instead of magic numbers. Small regression test added which
just makes sure that no exception is thrown when provided.

Result:
Fixes netty#15212

Co-authored-by: Michael Nitschinger <mnitschinger@apple.com>