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.content_type.should == "application/json"
> +            response_body = ActiveSupport::JSON.decode(response.body)
> +            response_body["errors"].should be_nil
> +            response_body["url"].should =~ /^http/
> +
> +            InfoRequest.count(:conditions => [
> +                "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"]

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; +            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
&gt; +            response_body[&quot;url&quot;].should =~ /^http/
&gt; +
&gt; +            InfoRequest.count(:conditions =&gt; [
&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;]
</pre>
<p>Prefer single-quoted strings when you don't need string interpolation or special symbols.</p>

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