Image title attribute missing

When outputting an image field that has a title attribute set, the title attribute is missing. For example:

"field_image" : {
  "src" : "http://example.com",
  "alt" : "My alternative text"
}

It'd be great if we could also get the title attribute, for example:

"field_image" : {
  "src" : "http://example.com",
  "alt" : "My alternative text",
  "title": "My title text"
}

Patch to come...