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
 (#1631)
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>  
> -        response_body = ActiveSupport::JSON.decode(response.body)
> -        response_body["errors"].should be_nil
> -        response_body["url"].should =~ /^http/
> +        it 'should update the status if a valid state is supplied' do
> +            # First we need an external request
> +            request_id = info_requests(:external_request).id
> +
> +            # Initially it has no incoming messages
> +            IncomingMessage.count(:conditions => ["info_request_id = ?", request_id]).should == 0
> +
> +            # Now add one
> +            sent_at = "2012-05-28T12:35:39+01:00"

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; -        response_body = ActiveSupport::JSON.decode(response.body)
&gt; -        response_body[&quot;errors&quot;].should be_nil
&gt; -        response_body[&quot;url&quot;].should =~ /^http/
&gt; +        it &#39;should update the status if a valid state is supplied&#39; do
&gt; +            # First we need an external request
&gt; +            request_id = info_requests(:external_request).id
&gt; +
&gt; +            # Initially it has no incoming messages
&gt; +            IncomingMessage.count(:conditions =&gt; [&quot;info_request_id = ?&quot;, request_id]).should == 0
&gt; +
&gt; +            # Now add one
&gt; +            sent_at = &quot;2012-05-28T12:35:39+01:00&quot;
</pre>
<p>Prefer single-quoted strings when you don't need string interpolation or special symbols.</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#r14195084">view it on GitHub</a>.<img alt="" height="1" src="https://github.com/notifications/beacon/510285__eyJzY29wZSI6Ik5ld3NpZXM6QmVhY29uIiwiZXhwaXJlcyI6MTcxOTMzMDkzMSwiZGF0YSI6eyJpZCI6MzUzMTE5NzR9fQ==--bedb6485a3153a06df6e9c7a824fa97936c9769a.gif" width="1" /></p>
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