Align rxFlowable nullability with the Reactive Streams spec

Resolves #347

Author: DmitriyZaitsev

reactive/kotlinx-coroutines-rx2/src/RxFlowable.kt

@@ -36,7 +36,7 @@ import kotlin.coroutines.*
  * @param block the coroutine code.
  */
 @ExperimentalCoroutinesApi
-public fun <T> CoroutineScope.rxFlowable(
+public fun <T: Any> CoroutineScope.rxFlowable(
     context: CoroutineContext = EmptyCoroutineContext,
     @BuilderInference block: suspend ProducerScope<T>.() -> Unit
 ): Flowable<T> = Flowable.fromPublisher(publish(newCoroutineContext(context), block = block))

reactive/kotlinx-coroutines-rx2/src/RxObservable.kt

@@ -52,7 +52,7 @@ private const val OPEN = 0        // open channel, still working
 private const val CLOSED = -1     // closed, but have not signalled onCompleted/onError yet
 private const val SIGNALLED = -2  // already signalled subscriber onCompleted/onError
 
-private class RxObservableCoroutine<T>(
+private class RxObservableCoroutine<T: Any>(
     parentContext: CoroutineContext,
     private val subscriber: ObservableEmitter<T>
 ) : AbstractCoroutine<Unit>(parentContext, true), ProducerScope<T>, SelectClause2<T, SendChannel<T>> {

reactive/kotlinx-coroutines-rx2/src/RxSingle.kt

@@ -36,7 +36,7 @@ public fun <T : Any> CoroutineScope.rxSingle(
     coroutine.start(CoroutineStart.DEFAULT, coroutine, block)
 }
 
-private class RxSingleCoroutine<T>(
+private class RxSingleCoroutine<T: Any>(
     parentContext: CoroutineContext,
     private val subscriber: SingleEmitter<T>
 ) : AbstractCoroutine<T>(parentContext, true) {