Allow serializing keys with \n in them

Use special quoted form

Closes https://github.com/alexcrichton/toml-rs/issues/185

src/ser.rs

@@ -107,9 +107,6 @@ pub enum Error {
     /// attempted where the key of a map was not a string.
     KeyNotString,
 
-    /// Keys in maps are not allowed to have newlines.
-    KeyNewline,
-
     /// Arrays in TOML must have a homogenous type, but a heterogeneous array
     /// was emitted.
     ArrayMixedType,
@@ -624,9 +621,6 @@ impl<'a, 'b> ser::SerializeMap for SerializeTable<'a, 'b> {
             SerializeTable::Table { ref mut key, .. } => {
                 key.truncate(0);
                 *key = input.serialize(StringExtractor)?;
-                if key.contains('\n') {
-                    return Err(Error::KeyNewline)
-                }
             }
         }
         Ok(())
@@ -1047,7 +1041,6 @@ impl fmt::Display for Error {
         match *self {
             Error::UnsupportedType => "unsupported Rust type".fmt(f),
             Error::KeyNotString => "map key was not a string".fmt(f),
-            Error::KeyNewline => "map keys cannot contain newlines".fmt(f),
             Error::ArrayMixedType => "arrays cannot have mixed types".fmt(f),
             Error::ValueAfterTable => "values must be emitted before tables".fmt(f),
             Error::DateInvalid => "a serialized date was invalid".fmt(f),
@@ -1064,7 +1057,6 @@ impl error::Error for Error {
         match *self {
             Error::UnsupportedType => "unsupported Rust type",
             Error::KeyNotString => "map key was not a string",
-            Error::KeyNewline => "map keys cannot contain newlines",
             Error::ArrayMixedType => "arrays cannot have mixed types",
             Error::ValueAfterTable => "values must be emitted before tables",
             Error::DateInvalid => "a serialized date was invalid",

tests/valid.rs

@@ -193,6 +193,9 @@ test!(example_bom,
 test!(datetime_truncate,
       include_str!("valid/datetime-truncate.toml"),
       include_str!("valid/datetime-truncate.json"));
+test!(key_quote_newline,
+      include_str!("valid/key-quote-newline.toml"),
+      include_str!("valid/key-quote-newline.json"));
 test!(table_array_nest_no_keys,
       include_str!("valid/table-array-nest-no-keys.toml"),
       include_str!("valid/table-array-nest-no-keys.json"));

tests/valid/key-quote-newline.json

@@ -0,0 +1,3 @@
+{
+    "\n": {"type": "integer", "value": "1"}
+}

tests/valid/key-quote-newline.toml

@@ -0,0 +1 @@
+"\n" = 1