[ruby/zlib] Zlib.gunzip should not fail with utf-8 strings

(https://github.com/ruby/zlib/pull/55)

zstream_discard_input was encoding and character-aware when given input is user-provided, so this discards `len` chars instead of `len` bytes.

Also Zlib.gunzip explains in its rdoc that it is equivalent with the following code, but this doesn't fail for UTF-8 String.

```ruby
string = %w[1f8b0800c28000000003cb48cdc9c9070086a6103605000000].pack("H*").force_encoding('UTF-8')
sio = StringIO.new(string)
p gz.read #=> "hello"
gz&.close
p Zlib.gunzip(string) #=> Zlib::DataError
```

Reported and discovered by eagletmt at https://twitter.com/eagletmt/status/1689692467929694209

https://github.com/ruby/zlib/commit/c5e58bc62a

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