Fixed bug for undefined method `string' for #<Mail::UnstructuredField:0x007fe62c5ad240>

Hi,
I found a bug which I really can't reproduce without giving someone my email server credentials (tried downloading the mail and parsing from there, but the bug didn't appear).

I got the following error when using the mail gem from rails:

```
/Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/mail-2.5.3/lib/mail/field.rb:165:in `method_missing': undefined method `string' for #<Mail::UnstructuredField:0x007fe62c5ad240> (NoMethodError)
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/mail-2.5.3/lib/mail/parts_list.rb:49:in `get_order_value'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/mail-2.5.3/lib/mail/parts_list.rb:38:in `block in sort!'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/mail-2.5.3/lib/mail/parts_list.rb:30:in `sort'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/mail-2.5.3/lib/mail/parts_list.rb:30:in `sort'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/mail-2.5.3/lib/mail/parts_list.rb:34:in `sort!'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/mail-2.5.3/lib/mail/message.rb:1690:in `ready_to_send!'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/mail-2.5.3/lib/mail/message.rb:1707:in `encoded'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/actionmailer-4.0.0.beta1/lib/action_mailer/base.rb:475:in `set_payload_for_mail'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/actionmailer-4.0.0.beta1/lib/action_mailer/base.rb:444:in `block in receive'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0.beta1/lib/active_support/notifications.rb:158:in `block in instrument'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0.beta1/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0.beta1/lib/active_support/notifications.rb:158:in `instrument'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/actionmailer-4.0.0.beta1/lib/action_mailer/base.rb:442:in `receive'
	from bin/receive_mail.rb:12:in `block in <top (required)>'
	from /Users/jojo/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/net/pop.rb:665:in `each'
	from /Users/jojo/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/net/pop.rb:665:in `each_mail'
	from bin/receive_mail.rb:11:in `<top (required)>'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/railties-4.0.0.beta1/lib/rails/commands/runner.rb:51:in `eval'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/railties-4.0.0.beta1/lib/rails/commands/runner.rb:51:in `<top (required)>'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/railties-4.0.0.beta1/lib/rails/commands.rb:86:in `require'
	from /Users/jojo/.rvm/gems/ruby-2.0.0-p0/gems/railties-4.0.0.beta1/lib/rails/commands.rb:86:in `<top (required)>'
	from bin/rails:4:in `require'
	from bin/rails:4:in `<main>'
```

So I found out in /lib/mail/parts_list.rb the following code
```ruby
    def get_order_value(part, order)
      if part.respond_to?(:content_type)
        order.index(part[:content_type].string.downcase) || 10000
      else
        10000
      end
    end
```

and decided to patch it with the following line
```ruby
if part.respond_to?(:content_type) and part[:content_type].respond_to?(:string)
```

It's not the best patch in the world, but at least it fixed the bug and it's a measure it should have been followed anyways.

I can imagine that the problem comes from the emails i'm passing, they're in german, so encoding might be the cause.

Also, this is the pretty print of the part we we're trying to send the ".string" message.

```
#<Mail::Field:0x007fe62c58dfa8
 @field=
  #<Mail::UnstructuredField:0x007fe62c5ad240
   @charset=#<Encoding:UTF-8>,
   @element=nil,
   @errors=
    [[#<ActiveSupport::Multibyte::Chars:0x007fe62c58d6e8
       @wrapped_string="Content-Type">,
      #<ActiveSupport::Multibyte::Chars:0x007fe62c58d490
       @wrapped_string="plain/text; name=\"sweets-online_AGB.txt\"">,
      #<Mail::Field::ParseError: Mail::ContentTypeElement can not parse |plain/text|
Reason was: Expected one of 7bit, 8bit, binary, quoted-printable, base64 at line 1, column 1 (byte 1) after >]],
   @length=nil,
   @name=
    #<ActiveSupport::Multibyte::Chars:0x007fe62c58d6e8
     @wrapped_string="Content-Type">,
   @tree=nil,
   @value="plain/text; name=\"sweets-online_AGB.txt\"">>
```

I hope it helps.