Porting some tests from 2.0, changed MemoryPoolWriter binary write funct...

...ion to WriteRawBytes
After some investigating I found that Xunit now has a `SynchronizationContext` that doesn't preserve culture like ASP.NET's. Previously Xunit didn't have any sync context. The test can be "fixed" by calling `task.PreserveCultureNotContext()` instead of `task.PreserveCulture()`.