Use null-terminated string in the tests when sending a raw topic

[minggangw]

Fix #771

[wayneparrott]

@minggangw I think it would be helpful to extend the comment on Publisher#publish() to note the null terminated string requirement. If you update the doc please update the publisher.d.ts api accordingly. https://github.com/RobotWebTools/rclnodejs/blob/31efa7c28e8d20bcd6fe1311fb10e9afa7bd63a7/lib/publisher.js#L41

Also I notice the raw publisher example does not use null terminated string. https://github.com/RobotWebTools/rclnodejs/blob/31efa7c28e8d20bcd6fe1311fb10e9afa7bd63a7/example/publisher-raw-message.js#L35

[minggangw]

The description of #771 may mislead the readers. I'd like to clarify it:

• The JavaScript doesn't use a null-terminated string as C does, so it's not necessary to add it.
• The buffer received comes from C functions, I guess it adds \0 recently which causes the failure. In order to pass the unit test, we have to use a null-terminated string when comparing.

I think we should update the comments for the subscription instead, what do you think?

[coveralls]

Coverage increased (+0.002%) to 91.789% when pulling e6c5d6e443a92c1fc0ec8f0113aa88d000ef1a60 on minggangw:fix-issue-771 into 31efa7c28e8d20bcd6fe1311fb10e9afa7bd63a7 on RobotWebTools:develop.