Add key/customerKey for ETTriggeredSendSummary

[algra]

This Scala code:

    val expression = new ETExpression
    expression.setProperty("customerKey")
    expression.setOperator(Operator.EQUALS)

    expression.addValue("my-customer-key")

    val filter = new ETFilter
    filter.addProperty("customerKey")
    filter.addProperty("queued")
    filter.addProperty("sent")
    filter.addProperty("bounces")
    filter.setExpression(expression)

    val response = client.retrieve(classOf[ETTriggeredSendSummary], filter)
    // do stuff with response

is working without any problems, but it seems returned value is getting lost because ETTriggeredSendSummary class does not have key or customerKey properties. I'm 100% sure value is returned by API, because I'm getting it if I write code for the same purpose using .NET SOAP implementation.

If I'm understanding it correctly, you just need to add something like this: https://github.com/salesforce-marketingcloud/FuelSDK-Java/blob/master/src/main/java/com/exacttarget/fuelsdk/ETTriggeredEmail.java#L80-L82 and this: https://github.com/salesforce-marketingcloud/FuelSDK-Java/blob/master/src/main/java/com/exacttarget/fuelsdk/ETTriggeredEmail.java#L150-L162

to this class: https://github.com/salesforce-marketingcloud/FuelSDK-Java/blob/master/src/main/java/com/exacttarget/fuelsdk/ETTriggeredSendSummary.java#L22

and it should work. Would you, guys, please, do it? I can also prepare a pull request for this if you want.