Optimize number of DATA frames for unary requests

Resolves #10

Author: ejona86

core/src/main/java/com/google/net/stubby/ChannelImpl.java

@@ -238,11 +238,14 @@ public void running() {}
   private class CallImpl<ReqT, RespT> extends Call<ReqT, RespT> {
     private final MethodDescriptor<ReqT, RespT> method;
     private final SerializingExecutor callExecutor;
+    private final boolean unaryRequest;
     private ClientStream stream;
 
     public CallImpl(MethodDescriptor<ReqT, RespT> method, SerializingExecutor executor) {
       this.method = method;
       this.callExecutor = executor;
+      this.unaryRequest = method.getType() == MethodType.UNARY
+          || method.getType() == MethodType.SERVER_STREAMING;
     }
 
     @Override
@@ -308,7 +311,12 @@ public void sendPayload(ReqT payload) {
           cancel();
         }
       }
-      stream.flush();
+      // For unary requests, we don't flush since we know that halfClose should be coming soon. This
+      // allows us to piggy-back the END_STREAM=true on the last payload frame without opening the
+      // possibility of broken applications forgetting to call halfClose without noticing.
+      if (!unaryRequest) {
+        stream.flush();
+      }
     }
 
     private class ClientStreamListenerImpl implements ClientStreamListener {