Encode control characters with hex not decimal

Fix https://github.com/alexcrichton/toml-rs/issues/178

src/ser.rs

@@ -290,7 +290,7 @@ impl<'a> Serializer<'a> {
                 '\u{22}' => drop(write!(self.dst, "\\\"")),
                 '\u{5c}' => drop(write!(self.dst, "\\\\")),
                 c if c < '\u{1f}' => {
-                    drop(write!(self.dst, "\\u{:04}", ch as u32))
+                    drop(write!(self.dst, "\\u{:04X}", ch as u32))
                 }
                 ch => drop(write!(self.dst, "{}", ch)),
             }

tests/valid/unicode-escape.json

@@ -1,4 +1,5 @@
 {
+    "answer1": {"type": "string", "value": "\u000B"},
     "answer4": {"type": "string", "value": "\u03B4α"},
     "answer8": {"type": "string", "value": "\u03B4β"}
 }

tests/valid/unicode-escape.toml

@@ -1,2 +1,3 @@
+answer1 = "\u000B"
 answer4 = "\u03B4α"
 answer8 = "\U000003B4β"