need to have test for request batching

[alexander-fenster]

As reported in #835, we lost the batching functionality because of a bug in the generator. Unfortunately, the tests did not catch it. We need to cover this piece of functionality with tests (possibly mocking the part that makes an actual request with sinon, and checking how many times it is called).

[losalex]

@alexander-fenster , is this feature related to this library or needs to be implemented in gapic layer?

[alexander-fenster]

@losalex Logging library is the only library that makes use of the batching feature, so it could go either way, but it will be easier to implement it here in this library. Then it can be backported to the generator if needed.