zlib strategy constants not strictly enforced

[Trott]

• Version: v8.0.0-pre
• Platform: Darwin Kernel Version 16.3.0
• Subsystem: zlib

zlib.constant.Z_RLE is 3 but code will happily accept "3" and work just fine.

Either we should add a test to confirm that strings continue to work, or else strictly enforce that only exactly the constants provided by zlib.constants are acceptable.

[Trott]

#10933 and #10934 are the two possible approaches to this issue.