check writer level when use NewWriterLevel

[heartlock]

Signed-off-by: luke chenjinguang@huawei.com

[heartlock]

@andybalholm please have a look. I want to import this package to kubernetes. But there are still some small flaws. if you want to know more info about this thing, please see kubernetes/enhancements#3235

[andybalholm]

Adding an error return to NewWriterLevel is an API change. Doing that would require releasing a new major version of the package.

Instead, we should replace levels less than BestSpeed with BestSpeed, and replace levels higher than BestCompression with BestCompression. This should be done in NewWriterOptions rather than in NewWriterLevel, to take care of both entry points at once.

[heartlock]

IMO, users should know this error just like gzip lib.

[andybalholm]

Yes, that's a fine API design. But it's not worth a breaking change.