Cloud format propagator parsing drops trace options

[c24t]

See #464 (comment).

>>> middleware.propagator.to_headers(SpanContext(trace_options=TraceOptions(0)))
{'X-Cloud-Trace-Context': 'd5e525e45bbe4d25b571fa0988749117/None;o=0'}

>>> middleware.propagator.from_headers({'X-Cloud-Trace-Context': 'd5e525e45bbe4d25b571fa0988749117/None;o=0'})
SpanContext(trace_id=d5e525e45bbe4d25b571fa0988749117, span_id=None, trace_options=TraceOptions(enabled=True), tracestate=None)