Fix content-type checker

[r4do]

When having type="text/html" set inside content-type, it's being incorrectly detected as text/html.

Example: valid header Content-Type: multipart/related; type="text/html" being detected as pure HTML, causes an error

     Failure/Error: @premailer ||= CustomizedPremailer.new(html_part.decoded)

     NoMethodError:
       Can not decode an entire message, try calling #decoded on the various fields and body or parts if it is a multipart message.

               raise NoMethodError, 'Can not decode an entire message, try calling #decoded on the various fields and body or parts if it is a multipart message.'
               ^^^^^