Add request state updating functionality to API

WIP

Adds a new `update_state` method to the API that uses the existing API key mechanism to send a status update to a request created via an external API request. Intended usage - foi-register end-users confirming whether the information they requested has been disclosed ([the parallel work on foi-register can be found here](https://github.com/mysociety/foi-register/tree/issues/169-ask-user-to-confirm-request-state)). Status description mismatches should be handled on the foi-register side but unexpected status values are rejected with a 500 error by the API.

Adds an optional parameter to the pre-existing `add_correspondence` API method to allow it to update status (intended use - updates from a public body) if a `state` value is supplied. Again, the foi-register code is responsible for handling state description mismatches but in this case the API deals with erroneous data by ignoring that part of the request.

Apologies for having restructured `api_controller_spec` but I kept losing track of which API methods the various tests were targeting. I will be happy to change it back before finalising the Pull Request if I've caused trouble.
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Subject: Re: [alaveteli] Add request state updating functionality to API
 (#1631)
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> +            # First we need an external request
> +            request_id = info_requests(:external_request).id
> +
> +            # Initially it has no incoming messages
> +            IncomingMessage.count(:conditions => ["info_request_id = ?", request_id]).should == 0
> +
> +            # Now add one
> +            sent_at = "2012-05-28T12:35:39+01:00"
> +            response_body = "Thank you for your request for information, which we are handling in accordance with the Freedom of Information Act 2000. You will receive a response within 20 working days or before the next full moon, whichever is sooner.\n\nYours sincerely,\nJohn Gandermulch,\nExample Council FOI Officer\n"
> +            post :add_correspondence,
> +                 :k => public_bodies(:geraldine_public_body).api_key,
> +                 :id => request_id,
> +                 :correspondence_json => {
> +                     "direction" => "response",
> +                     "sent_at" => sent_at,
> +                     "body" => response_body

Prefer single-quoted strings when you don't need string interpolation or special symbols.

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<p>In spec/controllers/api_controller_spec.rb:</p>
<pre style='color:#555'>&gt; +            # First we need an external request
&gt; +            request_id = info_requests(:external_request).id
&gt; +
&gt; +            # Initially it has no incoming messages
&gt; +            IncomingMessage.count(:conditions =&gt; [&quot;info_request_id = ?&quot;, request_id]).should == 0
&gt; +
&gt; +            # Now add one
&gt; +            sent_at = &quot;2012-05-28T12:35:39+01:00&quot;
&gt; +            response_body = &quot;Thank you for your request for information, which we are handling in accordance with the Freedom of Information Act 2000. You will receive a response within 20 working days or before the next full moon, whichever is sooner.\n\nYours sincerely,\nJohn Gandermulch,\nExample Council FOI Officer\n&quot;
&gt; +            post :add_correspondence,
&gt; +                 :k =&gt; public_bodies(:geraldine_public_body).api_key,
&gt; +                 :id =&gt; request_id,
&gt; +                 :correspondence_json =&gt; {
&gt; +                     &quot;direction&quot; =&gt; &quot;response&quot;,
&gt; +                     &quot;sent_at&quot; =&gt; sent_at,
&gt; +                     &quot;body&quot; =&gt; response_body
</pre>
<p>Prefer single-quoted strings when you don't need string interpolation or special symbols.</p>

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