mechanize：Mechanize::FileResponse#[]

>> require 'mechanize/file_response'
=> true
>> x = Mechanize::FileResponse.new("foo")
=> #<Mechanize::FileResponse:0xb7c72ce0 @file_path="foo">
>> x['content-type']
=> nil
>> x = Mechanize::FileResponse.new("foo.html")
=> #<Mechanize::FileResponse:0xb7c651f8 @file_path="foo.html">
>> x['content-type']
=> "text/html"
>> x['a']
=> nil
>> x = Mechanize::FileResponse.new("foo.xhtml")
=> #<Mechanize::FileResponse:0xb7c564a0 @file_path="foo.xhtml">
>> x['content-type']
=> "text/html"
>> x['a']
=> nil
>> x = Mechanize::FileResponse.new("foo_dir")
=> #<Mechanize::FileResponse:0xb7c48904 @file_path="foo_dir">
>> x['content-type']
=> "text/html"