Fix sequence sometimes not turning into a map (#450)

Previously, just referencing the next element in the sequence (and so constructing it, as an undefined element) would allow you to skip defining an element without turning the sequence into a map. E.g:

node[0] = "foo"; // sequence of size 1
node[1]; // sequence of size 1, with an undefined element at 1
node[2] = "bar"; // FIX: should be map of size 2 (since there's no element at index 1)

include/yaml-cpp/node/detail/impl.h

@@ -32,7 +32,7 @@ struct get_idx<Key,
 
   static node* get(std::vector<node*>& sequence, const Key& key,
                    shared_memory_holder pMemory) {
-    if (key > sequence.size())
+   if (key > sequence.size() || (key > 0 && !sequence[key-1]->is_defined()))
       return 0;
     if (key == sequence.size())
       sequence.push_back(&pMemory->create_node());

test/node/node_test.cpp

@@ -88,11 +88,22 @@ TEST(NodeTest, MapWithUndefinedValues) {
   EXPECT_EQ(2, node.size());
 }
 
+TEST(NodeTest, SeqIntoMap) {
+  Node node;
+  node[0] = "test";
+  node[1];
+  node[2] = "value";
+  EXPECT_TRUE(node.IsMap());
+  EXPECT_EQ("test", node[0].as<std::string>());
+  EXPECT_EQ("value", node[2].as<std::string>());
+  EXPECT_EQ(2, node.size());
+}
+
 TEST(NodeTest, RemoveUnassignedNode) {
   Node node(NodeType::Map);
   node["key"];
   node.remove("key");
-  EXPECT_EQ(node.size(), 0);
+  EXPECT_EQ(0, node.size());
 }
 
 TEST(NodeTest, MapForceInsert) {