title: Updating Contentful entry with a link/reference field
body: <p>The Contentful documentation is pretty sparse. I want to update a field on an entry that is a Link to another entry ("Reference" is what they call it in the CMS UI).</p>
  
  <p>Does anyone know what format they want this in when using the Contentful Management API?</p>
  
  <pre><code>type  ||= @mgmt.content_types.find(
    ENV['CONTENTFUL_SPACE_ID'], 'comments'
  )
  
  entry = type.entries.create(
    title: params[:title],
    article: params[:article]
  )
  </code></pre>
  
  <p>Where <code>:article</code> is the entry ID as a string. This returns:</p>
  
  <pre><code>undefined method `id' for "7L4IpEtsdffds8EsmoWMGIgy":String
  Extracted source (around line #74):
  
  #72         case field.type
  #73         when ContentType::LINK then
  *74           { sys: { type: field.type, linkType: field.link_type, id: attribute.id } } if attribute
  #75         when ContentType::ARRAY then
  #76           parse_fields_array(attribute)
  #77         when ContentType::LOCATION then
  </code></pre>
  
  <p>I've also tried to replace <code>params[:article]</code> with <code>@client.entry(params[:article])</code> thinking they may want the whole object but it returns the same error only it sees the same argument as an empty string.</p>
  
  <p>I have also tried this upon @DavidLitvak's suggestion:</p>
  
  <pre><code>link = Contentful::Management::Link.new({
    sys: {
      type: 'Link',
      linkType: 'Entry',
      id: params[:article]
    }
  })
  
  entry = type.entries.create(
    title: params[:title],
    article: link
  )
  </code></pre>
  
  <p>And although this does not throw an error, the article field shows up without an entry while the title field is populated.</p>
  
  <p>Also note that I am using the Ruby gem: <code>contentful-management</code>.</p>