Add conditional import for ByteReadChannel in MockEngine test code

[UnlikeMars]

Currently, the test code generation imports kotlinx.coroutines.experimental.io.*, required by ByteReadChannel for all Ktor versions. From ktor 1.3.0 the API changed and ByteReadChannel is now located under io.ktor.utils.io.* . As a result, generated Ktor projects (version >= 1.3.0) using the Mock HttpClient Engine do not compile.

This PR addresses #27 and points ByteReadChannel to the correct import depending on the selected Ktor version.