Added missing awaits to StreamWriter

The modified methods were added by @marek-safar in 185743cacd6f0f54caff73a87609bf603706e5ff, but it seems the code is missing `await`s in two places. The result would be that the returned `Task` would be completed too early.

In the case these two lines really shouldn't have `await`s, then I believe it doesn't make sense to use `ConfigureAwait()` on them and I think a comment about that should be added.

I didn't actually encounter any problem because of this, I just noticed warnings this caused in VS.