Avoid Enum.join when working with raw binaries

[jbowtie]

In Elixir 1.6 two tests fail -- this is because Enum.join is converting items to UTF-8 strings.

This pull request uses iodata_to_binary and list_to_bin instead of Enum.join when calling zlib since we are working with raw binary data.

[altyaper]

I tested this fix and it works 👍 thanks @jbowtie