Fix NetworkTable::ContainsKey.

Previously this would always return false due to how explicit bool is
evaluated in a return context.

Also add a test for this function.

Reported by: jcreigh

test/unit/NetworkTableTest.cpp

@@ -0,0 +1,20 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2015. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "networktables/NetworkTable.h"
+
+#include "gtest/gtest.h"
+
+class NetworkTableTest : public ::testing::Test {};
+
+TEST_F(NetworkTableTest, ContainsKey) {
+  auto nt = NetworkTable::GetTable("containskey");
+  ASSERT_FALSE(nt->ContainsKey("testkey"));
+  nt->PutNumber("testkey", 5);
+  ASSERT_TRUE(nt->ContainsKey("testkey"));
+}
+