Add support for boolean values

Merge #5 a=@mre r=@nox
________________________________________________________________________

src/ser/value.rs

@@ -51,8 +51,8 @@ impl<'key, 'target, Target> Serializer
     type StructState = ();
     type StructVariantState = ();
 
-    fn serialize_bool(&mut self, _v: bool) -> Result<(), Error> {
+    fn serialize_bool(&mut self, v: bool) -> Result<(), Error> {
-        Err(Error::unsupported_value())
+        self.append_pair(if v { "true" } else { "false" })
     }
 
     fn serialize_isize(&mut self, v: isize) -> Result<(), Error> {

tests/test_serialize.rs

@@ -25,3 +25,27 @@ fn serialize_option_map_string() {
         serde_urlencoded::to_string(params),
         Ok("first=hello&last=world".to_owned()));
 }
+
+#[test]
+fn serialize_option_map_bool() {
+    let params = &[
+        ("one", Some(true)),
+        ("two", Some(false))
+    ];
+
+    assert_eq!(
+        serde_urlencoded::to_string(params),
+        Ok("one=true&two=false".to_owned()));
+}
+
+#[test]
+fn serialize_map_bool() {
+    let params = &[
+        ("one", true),
+        ("two", false)
+    ];
+
+    assert_eq!(
+        serde_urlencoded::to_string(params),
+        Ok("one=true&two=false".to_owned()));
+}