Escape string containing colon

I don't know whether this is always necessary but it is required for
correctly serializing `["x: %"]`. If we serialize this without quotes to
`[x: %]` then the result is not valid YAML.

parser/src/emitter.rs

@@ -301,11 +301,12 @@ fn need_quotes(string: &str) -> bool {
     string == ""
         || need_quotes_spaces(string)
         || string.starts_with(|character: char| match character {
-            ':' | '&' | '*' | '?' | '|' | '-' | '<' | '>' | '=' | '!' | '%' | '@' => true,
+            '&' | '*' | '?' | '|' | '-' | '<' | '>' | '=' | '!' | '%' | '@' => true,
             _ => false,
         })
         || string.contains(|character: char| match character {
-            '{'
+            ':'
+            | '{'
             | '}'
             | '['
             | ']'

parser/tests/test_round_trip.rs

@@ -0,0 +1,23 @@
+extern crate yaml_rust;
+
+use yaml_rust::{Yaml, YamlEmitter, YamlLoader};
+
+fn test_round_trip(original: &Yaml) {
+    let mut out = String::new();
+    YamlEmitter::new(&mut out).dump(original).unwrap();
+    let documents = YamlLoader::load_from_str(&out).unwrap();
+    assert_eq!(documents.len(), 1);
+    assert_eq!(documents[0], *original);
+}
+
+#[test]
+fn test_escape_character() {
+    let y = Yaml::String("\x1b".to_owned());
+    test_round_trip(&y);
+}
+
+#[test]
+fn test_colon_in_string() {
+    let y = Yaml::String("x: %".to_owned());
+    test_round_trip(&y);
+}