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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> -
> -        response.should be_success
> -        response.should render_template("api/request_events")
> -        assigns[:events].size.should > 0
> -        assigns[:events].each do |event|
> -            event.created_at.should >= Date.new(2010, 1, 1)
> +        it 'should show a JSON feed of new request events' do
> +            get :body_request_events,
> +                :id => public_bodies(:geraldine_public_body).id,
> +                :k => public_bodies(:geraldine_public_body).api_key,
> +                :feed_type => 'json'
> +
> +            response.should be_success
> +            assigns[:events].size.should > 0
> +            assigns[:events].each do |event|
> +                event.info_request.public_body.should == public_bodies(:geraldine_public_body)

Line is too long. [94/80]

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<p>In spec/controllers/api_controller_spec.rb:</p>
<pre style='color:#555'>&gt; -
&gt; -        response.should be_success
&gt; -        response.should render_template(&quot;api/request_events&quot;)
&gt; -        assigns[:events].size.should &gt; 0
&gt; -        assigns[:events].each do |event|
&gt; -            event.created_at.should &gt;= Date.new(2010, 1, 1)
&gt; +        it &#39;should show a JSON feed of new request events&#39; do
&gt; +            get :body_request_events,
&gt; +                :id =&gt; public_bodies(:geraldine_public_body).id,
&gt; +                :k =&gt; public_bodies(:geraldine_public_body).api_key,
&gt; +                :feed_type =&gt; &#39;json&#39;
&gt; +
&gt; +            response.should be_success
&gt; +            assigns[:events].size.should &gt; 0
&gt; +            assigns[:events].each do |event|
&gt; +                event.info_request.public_body.should == public_bodies(:geraldine_public_body)
</pre>
<p>Line is too long. [94/80]</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#r14195054">view it on GitHub</a>.<img alt="" height="1" src="https://github.com/notifications/beacon/510285__eyJzY29wZSI6Ik5ld3NpZXM6QmVhY29uIiwiZXhwaXJlcyI6MTcxOTMzMDkyNywiZGF0YSI6eyJpZCI6MzUzMTE5NzR9fQ==--f84597d265d4889123fb2a3ecce125a31003e32c.gif" width="1" /></p>
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