Update SendGridEmail's content field to reflect the API

[jkloo]

According to the docs the content field needs to look like this

{
  // ...

  "content": [
    {
      "type": "text/html",
      "value": "Email Content"
    }
  ]

  // ...
}

rather than how it is in the current implementation

{
  // ...

  "content": [
    { "text/html": "Email Content"}
  ],

  // ...
}