title: Reading array from boost::property_tree comes up blank
body: <p>I am attempting to read array data from a <code>boost::property_tree</code> using the method shown <a href="https://stackoverflow.com/questions/4986052/boost-property-tree-working-with-simple-arrays-or-containers">in this question</a>.  In that example, the array is first read as a string, converted to a string stream, then read into an array.  While implementing that solution, I have noticed that my strings are coming up empty.  </p>
  
  <p>Example input (json):</p>
  
  <pre><code>"Object1"
  {
    "param1" : 10.0,
    "initPos" :
    {
      "":1.0,  
      "":2.0, 
      "":5.0 
    },
    "initVel" : [ 0.0, 0.0, 0.0 ]
  }
  </code></pre>
  
  <p>Both of these array notations are interpreted as arrays by the boost json parser.  I am confident that the data is present int the property tree, because on invocation of the json writer, the array data is present in the output.  </p>
  
  <p>This is an example of what is failing:</p>
  
  <pre><code>std::string paramName = "Object1.initPos";
  tempParamString = _runTree.get&lt;std::string&gt;(paramName,"Not Found");
  std::cout &lt;&lt; "Value: " &lt;&lt; tempParamString &lt;&lt; std::endl;
  </code></pre>
  
  <p>When <code>paramName</code> is <code>"Object1.param1"</code> I get "10.0" output as a string,
  When <code>paramName</code> is <code>"Object1.initPos"</code> I get an empty string,
  If <code>paramName</code> is something that does not exist in the tree, <code>"Not Found"</code> is returned.</p>