NoMethodError: undefined method `starts_with?' for nil:NilClass

The error occurs when using Pony.

```ruby
    Pony.mail({
      :to => 'email@domain.com',
      :from => 'noreply@domain.com',
      :subject => "Testing",
      :body => "This is just a test. Ignore it :)",
      :via => :smtp,
      :via_options => {
        :address              => 'smtp.gmail.com',
        :port                 => '587',
        :enable_starttls_auto => true,
        :user_name            => 'someone@gmail.com',
        :password             => 'very_secret',
        :authentication       => :plain
      }
    })
```

Just making sure ```part.content_type``` exists before calling ```.starts_with?(m)```