Headers with underscore are not accessible

[grosser]

>> Mail.new("X-Spam_score: 111")['X-Spam_Score']
=> nil

fix:

# every access to header is dasherized, so we have to dasherize when storing a field
# https://github.com/mikel/mail/issues/743
module HeaderWithUnderscore
  def initialize(*args)
    super
    @name = @name.gsub('_', '-')
  end
end
Mail::Field.prepend(HeaderWithUnderscore)

after:

Mail.new("X-Spam_score: 111")['X-Spam_Score']
=> #<Mail::Field 0x7f8f12bf0858 @charset="UTF-8" @name="X-Spam-score" @raw_value="X-Spam_score: 111" @value=nil>