SRVRecord doesn't allow updating

[regner]

When you try and update the value for a SRV record, say for example the target property, it doesn't properly build out the request.

https://github.com/dyninc/dyn-python/blob/master/dyn/tm/records.py#L2989

Here it sets the target API arg, but doesn't set any of the rest. The _post method will fully set all the API args, but that is only when creating a new SRV record.