Fix Internal Server Error (500) while null predicates.

After upgrading to node v.0.11.14, attributes passed to predicate
operators could have null values. In that case, NullPointerException is
thrown and HTTP error 500 is returned.

Test cases:

* *query* attribute is missing in HTTP call, while it is expected in predicate:

    curl -X DELETE http://localhost:4545/customer

    "equals": {
      "method": "DELETE",
      "path": "/customer",
      "query": {
        "id": "1001"
      }
    }

* *query* attribute is not expected in predicate and is not provided in HTTP call:

  curl -X PUT -d@data.json http://localhost:4545/customer

  "equals": {
    "method": "PUT",
    "path": "/customer"
  }
Merge pull request #53 from zedar/master

Fix Internal Server Error (500) if predicate attributes have null value.