5.1 did not set brotli default quality to 4 correctly

[ctusch]

The format for the brotli options is wrong. So brotli quality level still remains at 11.

Since brotli is the default this took me forever to find the reason why requests took so long. Since the actual compression is native, koa-compress doesn't show up on the profiler or when timing the middleware.

There is already an open PR #165