Fix formatting when writing \ as a character.

src/emitterutils.cpp

@@ -380,6 +380,8 @@ bool WriteChar(ostream_wrapper& out, char ch) {
     out << "\"\\n\"";
   } else if (ch == '\b') {
     out << "\"\\b\"";
+  } else if (ch == '\\') {
+    out << "\"\\\\\"";
   } else if ((0x20 <= ch && ch <= 0x7e) || ch == ' ') {
     out << "\"" << ch << "\"";
   } else {

test/integration/emitter_test.cpp

@@ -970,6 +970,14 @@ TEST_F(EmitterTest, ValueOfDoubleQuote) {
   ExpectEmit("foo: \"\\\"\"");
 }
 
+TEST_F(EmitterTest, ValueOfBackslash) {
+  out << YAML::BeginMap;
+  out << YAML::Key << "foo" << YAML::Value << '\\';
+  out << YAML::EndMap;
+
+  ExpectEmit("foo: \"\\\\\"");
+}
+
 class EmitterErrorTest : public ::testing::Test {
  protected:
   void ExpectEmitError(const std::string& expectedError) {