* exporter(stackdriver): fix ExportSpans when ctx is not nil
- problem: if ctx is not, calling cancel() will panic
- nil Context already handled in newContextWithTimeout
- add with test
* update go.sum
* mock traceserver for traceclient
* make precommit
* - respect option.Context when get google cred and traceclient
* Revise with timeout, context
- rename blackbox tests package to stackdriver_test
- remove context is nil checking, add to initialization
- add tests for timeout
