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
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> +            # Add another, as a followup
> +            sent_at = "2012-05-29T12:35:39+01:00"
> +            followup_body = "Pls answer ASAP.\nkthxbye\n"
> +            post :add_correspondence,
> +                 :k => public_bodies(:geraldine_public_body).api_key,
> +                 :id => request_id,
> +                 :correspondence_json => {
> +                     "direction" => "request",
> +                     "sent_at" => sent_at,
> +                     "body" => followup_body
> +                 }.to_json
> +
> +            # Make sure it worked
> +            response.should be_success
> +            followup_messages = OutgoingMessage.all(
> +                :conditions => ["info_request_id = ? and message_type = 'followup'", request_id]

Line is too long. [96/80]

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<p>In spec/controllers/api_controller_spec.rb:</p>
<pre style='color:#555'>&gt; +            # Add another, as a followup
&gt; +            sent_at = &quot;2012-05-29T12:35:39+01:00&quot;
&gt; +            followup_body = &quot;Pls answer ASAP.\nkthxbye\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;request&quot;,
&gt; +                     &quot;sent_at&quot; =&gt; sent_at,
&gt; +                     &quot;body&quot; =&gt; followup_body
&gt; +                 }.to_json
&gt; +
&gt; +            # Make sure it worked
&gt; +            response.should be_success
&gt; +            followup_messages = OutgoingMessage.all(
&gt; +                :conditions =&gt; [&quot;info_request_id = ? and message_type = &#39;followup&#39;&quot;, request_id]
</pre>
<p>Line is too long. [96/80]</p>

<p style="font-size:small;-webkit-text-size-adjust:none;color:#666;">&mdash;<br>Reply to this email directly or <a href="https://github.com/mysociety/alaveteli/pull/1631/files#r14195027">view it on GitHub</a>.<img alt="" height="1" src="https://github.com/notifications/beacon/510285__eyJzY29wZSI6Ik5ld3NpZXM6QmVhY29uIiwiZXhwaXJlcyI6MTcxOTMzMDkyNCwiZGF0YSI6eyJpZCI6MzUzMTE5NzR9fQ==--51b631211e422f5f731b811192a460a533427738.gif" width="1" /></p>
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