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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> +                "public_body_id = ?",
> +                public_bodies(:geraldine_public_body).id]
> +            ).should == number_of_requests + 1
> +
> +            new_request = InfoRequest.find(response_body["id"])
> +            new_request.user_id.should be_nil
> +            new_request.external_user_name.should == request_data["external_user_name"]
> +            new_request.external_url.should == request_data["external_url"]
> +
> +            new_request.title.should == request_data["title"]
> +            new_request.last_event_forming_initial_request.outgoing_message.body.should == request_data["body"].strip
> +
> +            new_request.public_body_id.should == public_bodies(:geraldine_public_body).id
> +            new_request.info_request_events.size.should == 1
> +            new_request.info_request_events[0].event_type.should == 'sent'
> +            new_request.info_request_events[0].calculated_state.should == 'waiting_response'

Line is too long. [92/80]

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<p>In spec/controllers/api_controller_spec.rb:</p>
<pre style='color:#555'>&gt; +                &quot;public_body_id = ?&quot;,
&gt; +                public_bodies(:geraldine_public_body).id]
&gt; +            ).should == number_of_requests + 1
&gt; +
&gt; +            new_request = InfoRequest.find(response_body[&quot;id&quot;])
&gt; +            new_request.user_id.should be_nil
&gt; +            new_request.external_user_name.should == request_data[&quot;external_user_name&quot;]
&gt; +            new_request.external_url.should == request_data[&quot;external_url&quot;]
&gt; +
&gt; +            new_request.title.should == request_data[&quot;title&quot;]
&gt; +            new_request.last_event_forming_initial_request.outgoing_message.body.should == request_data[&quot;body&quot;].strip
&gt; +
&gt; +            new_request.public_body_id.should == public_bodies(:geraldine_public_body).id
&gt; +            new_request.info_request_events.size.should == 1
&gt; +            new_request.info_request_events[0].event_type.should == &#39;sent&#39;
&gt; +            new_request.info_request_events[0].calculated_state.should == &#39;waiting_response&#39;
</pre>
<p>Line is too long. [92/80]</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#r14195021">view it on GitHub</a>.<img alt="" height="1" src="https://github.com/notifications/beacon/510285__eyJzY29wZSI6Ik5ld3NpZXM6QmVhY29uIiwiZXhwaXJlcyI6MTcxOTMzMDkyMiwiZGF0YSI6eyJpZCI6MzUzMTE5NzR9fQ==--6ec95a47d795e36c0c8187fb2b761c690218a030.gif" width="1" /></p>
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