brotli compression can be enabled just like gzip compression by setting ``'tools.br.on': True,``

``tools.br`` may also be combined with ``'tools.gzip`` - the client can then decide which encoding is accepted.

If both ``gzip`` and ``br`` tools are enabled and both ``gzip`` and ``br`` are accepted encodings, ``br`` will be chosen over ``gzip`` - unless ``gzip`` has a higher ``qvalue``

Changed default compression level for brotli from 5 to 4

Fixed an issue where the wrong compression level might be used if both gzip and brotli compression are enabled.

Changed brotli context from 'TOOLS.BROTLI' to 'TOOLS.BR' to be consistent with the tool name.

Tests now use the correct default compression level for brotli (which was changed from 5 to 4 in a previous commit)