def api_eval(expr_str)
      body = ["ver:\"#{@haystack_version}\""]
      body << "expr"
      body << '"'+expr_str+'"'
      res = self.connection.post('eval') do |req|
        req.headers['Content-Type'] = 'text/plain'
        req.body = body.join("\n")
      end
      JSON.parse! res.body
    end
def api_eval(expr_str)
      body = ["ver:\"#{@haystack_version}\""]
      body << "expr"
      body << '"'+expr_str+'"'
      res = self.connection.post('eval') do |req|
        req.headers['Content-Type'] = 'text/plain'
        req.body = body.join("\n")
      end
      JSON.parse! res.body
    end
this function will post expr_str exactly as encoded