Combines the string representation of elements into a single string.
Each element is converted to a string using its
separator is provided, it is inserted between element string
The returned future is completed with the combined string when this stream is done.
If this stream emits an error, or the call to
the returned future is completed with that error,
and processing stops.
@override AsObservableFuture<String> join([String separator = ""]) => AsObservableFuture<String>(_stream.join(separator));