title: Multiple optional keys in couchbase
body: <p>my map function</p>
  
  <pre><code>function(doc, meta) {
    emit([doc.city, doc.region], doc.id);
  }
  </code></pre>
  
  <p>my query</p>
  
  <pre><code>?keys=[["New York", "Northeast"]]
  </code></pre>
  
  <p>this will emit: </p>
  
  <pre><code>{ 
    city: 'New York',
    region: Northeast
  }
  </code></pre>
  
  <p>If I want use the same map function getting only those cities from Northeast it won't work. is there any way to match only 1 of the keys ? Like...</p>
  
  <pre><code>?keys=[[null, "Northeast"]]
  </code></pre>
  
  <p>and outputing</p>
  
  <pre><code>[{ 
    city: 'New York',
    region: 'Northeast'
  },
  {
    city: 'Boston',
    region: 'Northeast'
  }]
  </code></pre>