title: Swift Dictionary: join key and value into string
body: <p>I'm looking for a best syntax for this:</p>
  
  <pre><code>        let responseParameters = ["keyA" : "valueA", "keyB" : "valueB"]
          var responseString = ""
          for (key, value) in responseParameters {
              responseString += "\(key):\(value)"
              if Array(responseParameters.keys).last != key {
                  responseString += "+"
              }
          }
          //responseString: keyA:valueA+keyB:valueB
  </code></pre>
  
  <p>Something like an array joinWithSeparator, using a flatMap or something like that. (study purpose)</p>