Improve Multipart messages parsing

[roxblnfk]

• Content-Disposition is optional field and the parser shouldn't fail on it (resolved in Better HTTP/SMTP Multipart parsing #128)
• Multipart data may be nested and contain dot at the end:
  
  https://stackoverflow.com/questions/3902455/mail-multipart-alternative-vs-multipart-mixed

  Header
  |From: email
  |To: email
  |MIME-Version: 1.0
  |Content-Type: multipart/mixed; boundary="boundary1";
  Message body
  |multipart/mixed --boundary1
  |--boundary1
  |   multipart/related --boundary2
  |   |--boundary2
  |   |   multipart/alternative --boundary3
  |   |   |--boundary3
  |   |   |text/plain
  |   |   |--boundary3
  |   |   |text/html
  |   |   |--boundary3--
  |   |--boundary2    
  |   |Inline image
  |   |--boundary2    
  |   |Inline image
  |   |--boundary2--
  |--boundary1    
  |Attachment1
  |--boundary1
  |Attachment2
  |--boundary1
  |Attachment3
  |--boundary1--
  |
  .
  

Related issue: buggregator/server#213