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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> +                     "direction" => "response",
> +                     "sent_at" => sent_at,
> +                     "body" => response_body
> +                 }.to_json,
> +                 :attachments => [
> +                     fixture_file_upload("/files/tfl.pdf")
> +                 ]
> +
> +            # And make sure it worked
> +            response.should be_success
> +            incoming_messages = IncomingMessage.all(:conditions => ["info_request_id = ?", request_id])
> +            incoming_messages.count.should == 1
> +            incoming_message = incoming_messages[0]
> +
> +            incoming_message.sent_at.should == Time.iso8601(sent_at)
> +            incoming_message.get_main_body_text_folded.should be_equal_modulo_whitespace_to(response_body)

Line is too long. [106/80]

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<p>In spec/controllers/api_controller_spec.rb:</p>
<pre style='color:#555'>&gt; +                     &quot;direction&quot; =&gt; &quot;response&quot;,
&gt; +                     &quot;sent_at&quot; =&gt; sent_at,
&gt; +                     &quot;body&quot; =&gt; response_body
&gt; +                 }.to_json,
&gt; +                 :attachments =&gt; [
&gt; +                     fixture_file_upload(&quot;/files/tfl.pdf&quot;)
&gt; +                 ]
&gt; +
&gt; +            # And make sure it worked
&gt; +            response.should be_success
&gt; +            incoming_messages = IncomingMessage.all(:conditions =&gt; [&quot;info_request_id = ?&quot;, request_id])
&gt; +            incoming_messages.count.should == 1
&gt; +            incoming_message = incoming_messages[0]
&gt; +
&gt; +            incoming_message.sent_at.should == Time.iso8601(sent_at)
&gt; +            incoming_message.get_main_body_text_folded.should be_equal_modulo_whitespace_to(response_body)
</pre>
<p>Line is too long. [106/80]</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#r14195044">view it on GitHub</a>.<img alt="" height="1" src="https://github.com/notifications/beacon/510285__eyJzY29wZSI6Ik5ld3NpZXM6QmVhY29uIiwiZXhwaXJlcyI6MTcxOTMzMDkyNiwiZGF0YSI6eyJpZCI6MzUzMTE5NzR9fQ==--843305172b17c3ff16cd966b8ccf2fc27c7e80dc.gif" width="1" /></p>
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