fixed an incorrect assumption about instance of IO

Symptom is an absence of output files. 
Found when tested the following code with SikuliX IDE
```ruby
    out_html = 'cucumber_result.html'
    args = [
      '--format', 'pretty',
      '--format', 'html',
      '--out', out_html
    ]
    require 'cucumber/rspec/disable_option_parser'
    require 'cucumber/cli/main'
    Cucumber::Cli::Main.new(args).execute!
```

Reason:
```ruby
str = '123'
p str.respond_to? :write  
# => false

Object.send('define_method', 'write'){  puts '***' }

p str.respond_to? :write  
# => true!!!
```