Add support for option parameters

Merge #4 a=@mre r=@nox
________________________________________________________________________

src/ser/value.rs

@@ -158,13 +158,17 @@ impl<'key, 'target, Target> Serializer
     }
 
     fn serialize_none(&mut self) -> Result<(), Error> {
-        Err(Error::unsupported_value())
+        if let Some(_) = self.key.take() {
+            Ok(())
+        } else {
+            Err(Error::no_key())
+        }
     }
 
-    fn serialize_some<T>(&mut self, _value: T) -> Result<(), Error>
+    fn serialize_some<T>(&mut self, value: T) -> Result<(), Error>
         where T: Serialize
     {
-        Err(Error::unsupported_value())
+        value.serialize(self)
     }
 
     fn serialize_seq(&mut self, _len: Option<usize>) -> Result<(), Error> {

tests/test_serialize.rs

@@ -0,0 +1,27 @@
+extern crate serde_urlencoded;
+
+#[test]
+fn serialize_option_map_int() {
+    let params = &[
+        ("first", Some(23)),
+        ("middle", None),
+        ("last", Some(42)),
+    ];
+
+    assert_eq!(
+        serde_urlencoded::to_string(params),
+        Ok("first=23&last=42".to_owned()));
+}
+
+#[test]
+fn serialize_option_map_string() {
+    let params = &[
+        ("first", Some("hello")),
+        ("middle", None),
+        ("last", Some("world")),
+    ];
+
+    assert_eq!(
+        serde_urlencoded::to_string(params),
+        Ok("first=hello&last=world".to_owned()));
+}