title: Artillery.io value obtained on 'capture' not available at 'expect'
body: <p>I'm quite new to the artillery.io and I'm getting a problem.
  I want to get a value from the JSON response and check it on the 'expect' clause, so I'm doing that like this:</p>
  
  <pre><code>config:
    target: 'https://api.duckduckgo.com'
    plugins:
        expect: {}
    phases:
      - duration: 1
        arrivalRate: 1
  scenarios:
     - flow:
        - get:
            url: "/?q=cao&amp;format=json"
            capture:
              - json: $RelatedTopics[0].FirstURL
                as: url
            expect:
              - statusCode: 200
              - equals:
                - "{{ url }}"
                - "https://duckduckgo.com/209/Cao_Wei"
        - log: "url is {{ url }}"
  </code></pre>
  
  <p>Then I call the artillery just like this:</p>
  
  <pre><code>artillery run duck.yml
  </code></pre>
  
  <p>Then I get something like this:</p>
  
  <pre><code>User variables:
       target : https://api.duckduckgo.com
       $environment : undefined
       $uuid : 0774da00-b6e6-415c-bd28-0ff4bedd07ec
    not ok equals , https://duckduckgo.com/209/Cao_Wei 
    expected: all values to be equal
         got: , https://duckduckgo.com/209/Cao_Wei
    Request params:
      https://api.duckduckgo.com/?q=cao&amp;format=json
      ""
  </code></pre>
  
  <p>I omitted the rest of the output cause it is too big.
  When it says "not ok equals , <a href="https://duckduckgo.com/209/Cao_Wei" rel="nofollow noreferrer">https://duckduckgo.com/209/Cao_Wei</a>" looks like to me that my <em>url</em> variable is undefined.</p>
  
  <p>Using the JSONPath on quotes like this "$RelatedTopics[0].FirstURL" gives the same problem.</p>
  
  <p>I installed the expect (<a href="https://artillery.io/docs/plugin-expectations-assertions/" rel="nofollow noreferrer">https://artillery.io/docs/plugin-expectations-assertions/</a>) plugin like this:</p>
  
  <pre><code>npm install -g artillery-plugin-expect
  </code></pre>
  
  <p>Please, what am I missing here?</p>