title: rmongodb $in query issues
body: <p>I have really enjoyed working with rmongodb and have figured out how to do a lot of different things/combination of queries but this one still stumps me. I am trying to get all of the documents that have the symbol: field equal to an element of a vector (named symbols in r).</p>
  
  <p>Query that works on my MongoDB DB that I am trying to replicate in r</p>
  
  <pre><code>{symbol: {"$in": ["SYMBOLA","SYMBOLB","SYMBOLC","SYMBOLD","SYMBOLE"]} }
  </code></pre>
  
  <p>In r...</p>
  
  <pre><code>symbols &lt;- list("SYMBOLA","SYMBOLB","SYMBOLC","SYMBOLD","SYMBOLE")
  mongo.find.all(mongo, namespace, query = list(symbol = list('$in' = symbols)))
  </code></pre>
  
  <p>Doesnt seem to work (since I am not used to the syntax I tried all sorts of combinations without luck. Feel like I am missing something here.</p>
  
  <p>Any help on how I can query specific documents based on a symbols vector in R would be a huge help.</p>
  
  <p>EDIT:
  Here is the print of the BSON:</p>
  
  <pre><code>symbol : 3   
          $in : 4      
              0 : 2    SYMBOLA
              1 : 2    SYMBOLB
              2 : 2    SYMBOLC
              3 : 2    SYMBOLD
              4 : 2    SYMBOLE
  </code></pre>