Update UnitTest of MapUtilTest.cpp

Summary:
1. Split the original simple test
2. Add unit test of function: get_ref_default

Closes #138

Test Plan: authored tested, contbuild

Reviewed By: markisaa@fb.com

Subscribers: folly-diffs@, yfeldblum

FB internal diff: D1878603

Signature: t1:1878603:1425073330:914a2365fa648142d11b41b387423d78e52f2e08

folly/test/MapUtilTest.cpp

@@ -21,12 +21,25 @@
 
 using namespace folly;
 
-TEST(MapUtil, Simple) {
+TEST(MapUtil, get_default) {
   std::map<int, int> m;
   m[1] = 2;
   EXPECT_EQ(2, get_default(m, 1, 42));
   EXPECT_EQ(42, get_default(m, 2, 42));
   EXPECT_EQ(0, get_default(m, 3));
+}
+
+TEST(MapUtil, get_ref_default) {
+  std::map<int, int> m;
+  m[1] = 2;
+  const int i = 42;
+  EXPECT_EQ(2, get_ref_default(m, 1, i));
+  EXPECT_EQ(42, get_ref_default(m, 2, i));
+}
+
+TEST(MapUtil, get_ptr) {
+  std::map<int, int> m;
+  m[1] = 2;
   EXPECT_EQ(2, *get_ptr(m, 1));
   EXPECT_TRUE(get_ptr(m, 2) == nullptr);
   *get_ptr(m, 1) = 4;