Saving Draft: convenience header & empty body

= Why

As a user, I want to create a mail draft (but empty body for the moment)

= How

Implement [`Email/set create` ](https://jmap.io/spec-mail.html#emailset)

[[ "Email/set", {
  "accountId": "ue150411c",
  "create": {
    "k192": {
      "mailboxIds": {
        "2ea1ca41b38e": true
      },
      "keywords": {
        "$seen": true,
        "$draft": true
      },
      "from": [{
        "name": "Joe Bloggs",
        "email": "joe@example.com"
      }],
      "subject": "World domination",
      "receivedAt": **"2018-07-10T01:03:11Z",
      "sentAt": "2018-07-10T11:03:11+10:00",
    }
  }
}, "0" ]]

Notes:

• omitting the body part for now (see #3910)
• not all headers are allowed for now, only the convenience ones, as:
• `references` | defaults to null
• `inReplyTo` | defaults to null
• `sender` | default to `from`
• `from` | defaults to null
• `to` | defaults to null
• `cc` | defaults to null
• `bcc` | defaults to null
• `replyTo` | defaults to `to`
• `subject` | defaults to null
• `sentAt` | defaults to `now`
• other parameters can set:
• `mailboxIds` must be set
• `keywords` | default is null
• `receivedAt` | default is time of creation on server
• server-set parameters:
• `id`
• `threadId`
• `size`

Restrictions:

• The headers property MUST NOT be given on either the top-level Email or an EmailBodyPart — the client must set each header field as an individual property.
• There MUST NOT be two properties that represent the same header field (e.g., header:from and from) within the Email or particular EmailBodyPart.
• Header fields MUST NOT be specified in parsed forms that are forbidden for that particular field.
• Header fields beginning with Content- MUST NOT be specified on the Email object, only on EmailBodyPart objects.

= Definition of Done

Write integration tests about email creation and error handlings.

Delegation needs to be handled.