Zlib generated does not include proper compression header

[otavio]

The generated fine does not include the referred compression level.

It should looks like:

Level | ZLIB  | GZIP 
  1   | 78 01 | 1F 8B 
  2   | 78 5E | 1F 8B 
  3   | 78 5E | 1F 8B 
  4   | 78 5E | 1F 8B 
  5   | 78 5E | 1F 8B 
  6   | 78 9C | 1F 8B 
  7   | 78 DA | 1F 8B 
  8   | 78 DA | 1F 8B 
  9   | 78 DA | 1F 8B 

However, the second byte is always 0x01. This happens using the default feature. Using zlib backend it works as expected.

Originally reported at: https://github.com/alexcrichton/flate2-rs/issues/225

[oyvindln]

This wasn't in the original miniz either, so we never bothered to implement it, but it should be easy enough if it's needed for anything.

[otavio]

This wasn't in the original miniz either, so we never bothered to implement it, but it should be easy enough if it's needed for anything.

It is so it mimics the zlib behavior. For now, I am using it but would prefer to use a pure Rust implementation instead.