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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> -        }
> +            request_data = {
> +                "title" => "Tell me about your chickens",
> +                "body" => "Dear Sir,\n\nI should like to know about your chickens.\n\nYours in faith,\nBob\n",
> +                "external_url" => "http://www.example.gov.uk/foi/chickens_23",
> +                "external_user_name" => "Bob Smith",
> +            }
> +
> +            post :create_request,
> +                 :k => public_bodies(:geraldine_public_body).api_key,
> +                 :request_json => request_data.to_json
> +            response.should be_success
> +
> +            response.content_type.should == "application/json"
> +            response_body = ActiveSupport::JSON.decode(response.body)
> +            response_body["errors"].should be_nil

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; -        }
&gt; +            request_data = {
&gt; +                &quot;title&quot; =&gt; &quot;Tell me about your chickens&quot;,
&gt; +                &quot;body&quot; =&gt; &quot;Dear Sir,\n\nI should like to know about your chickens.\n\nYours in faith,\nBob\n&quot;,
&gt; +                &quot;external_url&quot; =&gt; &quot;http://www.example.gov.uk/foi/chickens_23&quot;,
&gt; +                &quot;external_user_name&quot; =&gt; &quot;Bob Smith&quot;,
&gt; +            }
&gt; +
&gt; +            post :create_request,
&gt; +                 :k =&gt; public_bodies(:geraldine_public_body).api_key,
&gt; +                 :request_json =&gt; request_data.to_json
&gt; +            response.should be_success
&gt; +
&gt; +            response.content_type.should == &quot;application/json&quot;
&gt; +            response_body = ActiveSupport::JSON.decode(response.body)
&gt; +            response_body[&quot;errors&quot;].should be_nil
</pre>
<p>Prefer single-quoted strings when you don't need string interpolation or special symbols.</p>

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