Fix SendAsync on Unix with multiple buffers

In the implementation of Send{To}Async for a list of array segments, we try to do the send operation, and if it doesn't send the full amount (e.g. because it does one send that's less than the full amount and then tries to complete it but gets an EAGAIN), we then fall back to creating a SendOperation that stores the data to try again later.  But the number of bytes sent wasn't being copied into that SendOperation data structure, so although the data had been sent and although we were correctly updating the pointers to what to send next and when we were done sending, the actual total number of BytesTransferred at the end of the operation would be missing the sum from that initial successful set of synchronous sends prior to the EAGAIN.  The fix is simply to store the initial bytesSent into BytesTransferred, as is done for every other send operation on SocketAsyncContext.

Author: stephentoub

src/System.Net.Sockets/src/System/Net/Sockets/SocketAsyncContext.Unix.cs

@@ -1385,6 +1385,7 @@ public SocketError SendToAsync(IList<ArraySegment<byte>> buffers, SocketFlags fl
                     Flags = flags,
                     SocketAddress = socketAddress,
                     SocketAddressLen = socketAddressLen,
+                    BytesTransferred = bytesSent
                 };
 
                 bool isStopped;