Serialization error for Traces between PHP and Go daemon

[subnova]

The go daemon requires stackTrace to be an array of strings.

https://github.com/census-instrumentation/opencensus-php/blob/master/daemon/processor/local/helper_types.go#L37

However, the Span class in the PHP library sets the stackTrace to an array of objects.

https://github.com/census-instrumentation/opencensus-php/blob/master/src/Trace/Span.php#L226

DaemonClient should serialize the objects before sending them to the daemon.