Support for application/json in the default serializer

[dillonredding]

The default serializer could support serializing an Action with type 'application/json'. The resulting serialization would be a JSON object whose entries are the name-value pairs from each Field in the Action.

For example, these fields:

[
  { "name": "foo", "value": "bar" },
  { "name": "baz", "value": 42 },
  { "name": "qux", "value": true }
]

would serialize as:

{
  "foo": "bar",
  "baz": 42,
  "qux": true
}

Alternatively, the fields could simply be sent as is (as an array), but that scenario seems less desirable since the field's may contain a lot of metadata that the server does not need.