Use JsonBodyWriter directly rather than converting JSON to strings first

Change-Id: I2facf51d0bf9b6ea4ccb47b0ac4214af203982a1
diff --git a/web/api/src/main/java/org/onosproject/rest/FlowsWebResource.java b/web/api/src/main/java/org/onosproject/rest/FlowsWebResource.java
index 24ee73a..36afcce 100644
--- a/web/api/src/main/java/org/onosproject/rest/FlowsWebResource.java
+++ b/web/api/src/main/java/org/onosproject/rest/FlowsWebResource.java
@@ -65,7 +65,7 @@
             }
         }
 
-        return ok(root.toString()).build();
+        return ok(root).build();
     }
 
     /**
@@ -87,7 +87,7 @@
         for (final FlowEntry entry : deviceEntries) {
             flowsNode.add(flowEntryCodec.encode(entry, this));
         }
-        return ok(root.toString()).build();
+        return ok(root).build();
     }
 
     /**
@@ -113,6 +113,6 @@
                 flowsNode.add(flowEntryCodec.encode(entry, this));
             }
         }
-        return ok(root.toString()).build();
+        return ok(root).build();
     }
 }