Submit doesn't handle header name-value pairs array

[dillonredding]

Given an action that would generate a request body, the following:

submit(action, {
  requestInit: {
    headers: [['Foo', 'bar']]
  }
});

generates the following request:

POST /foo HTTP/1.1
Content-Type: application/x-www-form-urlencoded
0: Foo,bar

...

[dillonredding]

@miller79