Disable onCommit after first commit

At times OnCommittedResponseWrapper#onResponseCommitted() can be invoked
multiple times. For example, when flush is performed multiple times. This
means that the session can be written multiple times which is inefficient.
Instead, we should only save on the first update.

Fixes gh-57

spring-session/src/test/java/org/springframework/session/web/http/OnCommittedResponseWrapperTests.java

@@ -28,7 +28,7 @@ public class OnCommittedResponseWrapperTests {
     @Mock
     ServletOutputStream out;
 
-    HttpServletResponse response;
+    OnCommittedResponseWrapper response;
 
     boolean committed;
 
@@ -1069,4 +1069,20 @@ public class OnCommittedResponseWrapperTests {
 
         assertThat(committed).isTrue();
     }
+
+    @Test
+    public void bufferSizeCommitsOnce() throws Exception {
+        String expected = "1234567890";
+        when(response.getBufferSize()).thenReturn(expected.length());
+
+        response.getWriter().write(expected);
+
+        assertThat(committed).isTrue();
+
+        committed = false;
+
+        response.getWriter().write(expected);
+
+        assertThat(committed).isFalse();
+    }
 }