add missing const to fix `folly::toDynamic` on `std::vector<bool>`

Summary: [folly] add missing const to fix `folly::toDynamic` on `std::vector<bool>`

Reviewed By: ender-wieczorek, yfeldblum

Differential Revision: D5039403

fbshipit-source-id: edd052c7d1d832d424166cba15fcd9f4f8bd219c

folly/DynamicConverter.h

@@ -317,7 +317,7 @@ struct DynamicConstructor<
         dynamicconverter_detail::is_map<C>::value>::type> {
   static dynamic construct(const C& x) {
     dynamic d = dynamic::object;
-    for (auto& pair : x) {
+    for (const auto& pair : x) {
       d.insert(toDynamic(pair.first), toDynamic(pair.second));
     }
     return d;
@@ -335,7 +335,7 @@ struct DynamicConstructor<
         dynamicconverter_detail::is_range<C>::value>::type> {
   static dynamic construct(const C& x) {
     dynamic d = dynamic::array;
-    for (auto& item : x) {
+    for (const auto& item : x) {
       d.push_back(toDynamic(item));
     }
     return d;

folly/test/DynamicConverterTest.cpp

@@ -382,6 +382,12 @@ TEST(DynamicConverter, construct) {
                                dynamic::array(3, 4, 5));
     EXPECT_EQ(d, toDynamic(c));
   }
+
+  {
+    vector<bool> vb{true, false};
+    dynamic d = dynamic::array(true, false);
+    EXPECT_EQ(d, toDynamic(vb));
+  }
 }
 
 TEST(DynamicConverter, errors) {