change encoding to ascii-8bit in zlib.rb

[FieryRMS]

This PR fixes the following issue.

meta XML:com.adobe.xmp..
/var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/checker/zlib.rb:26:in `index': incompatible character encodings: UTF-8 and ASCII-8BIT (Encoding::CompatibilityError)
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/checker/zlib.rb:26:in `check_data'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/checker.rb:395:in `data2result'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/checker.rb:314:in `process_result'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/checker.rb:185:in `block in check_metadata'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/checker.rb:182:in `each'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/checker.rb:182:in `check_metadata'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/checker.rb:77:in `check'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/cli/cli.rb:258:in `check'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/cli/cli.rb:172:in `block (2 levels) in run'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/cli/cli.rb:168:in `each'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/cli/cli.rb:168:in `block in run'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/cli/cli.rb:161:in `each'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/cli/cli.rb:161:in `each_with_index'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg/cli/cli.rb:161:in `run'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/lib/zsteg.rb:26:in `run'
        from /var/lib/gems/3.1.0/gems/zsteg-0.2.13/bin/zsteg:8:in `<top (required)>'
        from /usr/local/bin/zsteg:25:in `load'
        from /usr/local/bin/zsteg:25:in `<main>'

Sample file electricsoldiers.png from DEADFACE CTF 2024

_{P.S. I do not know ruby, I just did a quick google search to fix this}

[imadogB]

Nice