Fix MockWebServer websockets to be thread safe. (square#3567)

Closing the sink while another thread is writing the sink is a
potential source for races.

Closes: square#3138

Author: swankjesse

mockwebserver/src/main/java/okhttp3/mockwebserver/MockWebServer.java

@@ -465,8 +465,6 @@ public void processConnection() throws Exception {
               + " didn't make a request");
         }
 
-        source.close();
-        sink.close();
         socket.close();
         openClientSockets.remove(socket);
       }
@@ -681,7 +679,6 @@ private void handleWebSocketUpgrade(Socket socket, BufferedSource source, Buffer
     } catch (IOException e) {
       webSocket.failWebSocket(e, null);
     } finally {
-      closeQuietly(sink);
       closeQuietly(source);
     }
   }

okhttp-tests/src/test/java/okhttp3/internal/ws/WebSocketHttpTest.java

@@ -322,7 +322,7 @@ public final class WebSocketHttpTest {
             assertNull(chain.request().body());
             Response response = chain.proceed(chain.request());
             assertEquals("Upgrade", response.header("Connection"));
-            assertTrue("", response.body().source().exhausted());
+            assertTrue(response.body().source().exhausted());
             interceptedCount.incrementAndGet();
             return response;
           }