Handle a key with length over 1024 as a long key. (#916)

Fixes #501

src/emitter.cpp

@@ -708,7 +708,7 @@ Emitter& Emitter::Write(const std::string& str) {
       Utils::ComputeStringFormat(str, m_pState->GetStringFormat(),
                                  m_pState->CurGroupFlowType(), stringEscaping == StringEscaping::NonAscii);
 
-  if (strFormat == StringFormat::Literal)
+  if (strFormat == StringFormat::Literal || str.size() > 1024)
     m_pState->SetMapKeyFormat(YAML::LongKey, FmtScope::Local);
 
   PrepareNode(EmitterNodeType::Scalar);

test/integration/emitter_test.cpp

@@ -266,6 +266,8 @@ TEST_F(EmitterTest, SimpleLongKey) {
 }
 
 TEST_F(EmitterTest, SingleLongKey) {
+  const std::string shortKey(1024, 'a');
+  const std::string longKey(1025, 'a');
   out << BeginMap;
   out << Key << "age";
   out << Value << "24";
@@ -273,9 +275,14 @@ TEST_F(EmitterTest, SingleLongKey) {
   out << Value << "5'9\"";
   out << Key << "weight";
   out << Value << 145;
+  out << Key << shortKey;
+  out << Value << "1";
+  out << Key << longKey;
+  out << Value << "1";
   out << EndMap;
 
-  ExpectEmit("age: 24\n? height\n: 5'9\"\nweight: 145");
+  ExpectEmit("age: 24\n? height\n: 5'9\"\nweight: 145\n" + shortKey +
+             ": 1\n? " + longKey + "\n: 1");
 }
 
 TEST_F(EmitterTest, ComplexLongKey) {