Do not forbid bool keys anymore

Floats are supported so...

src/ser/key.rs

@@ -46,10 +46,6 @@ impl<End, Ok> Sink for KeySink<End>
 {
     type Ok = Ok;
 
-    fn serialize_bool(self, _value: bool) -> Result<Ok, Error> {
-        Err(self.unsupported())
-    }
-
     fn serialize_static_str(self,
                             value: &'static str)
                             -> Result<Ok, Error> {

src/ser/part.rs

@@ -18,8 +18,6 @@ impl<S: Sink> PartSerializer<S> {
 pub trait Sink: Sized {
     type Ok;
 
-    fn serialize_bool(self, value: bool) -> Result<Self::Ok, Error>;
-
     fn serialize_static_str(self,
                             value: &'static str)
                             -> Result<Self::Ok, Error>;
@@ -48,7 +46,7 @@ impl<S: Sink> ser::Serializer for PartSerializer<S> {
     type SerializeStructVariant = VoidSerializer<S::Ok>;
 
     fn serialize_bool(self, v: bool) -> Result<S::Ok, Error> {
-        self.sink.serialize_bool(v)
+        self.sink.serialize_static_str(if v { "true" } else { "false" })
     }
 
     fn serialize_i8(self, v: i8) -> Result<S::Ok, Error> {

src/ser/value.rs

@@ -30,10 +30,6 @@ impl<'key, 'target, Target> Sink for ValueSink<'key, 'target, Target>
 {
     type Ok = ();
 
-    fn serialize_bool(self, value: bool) -> Result<Self::Ok, Error> {
-        self.serialize_static_str(if value { "true" } else { "false" })
-    }
-
     fn serialize_str(self, value: &str) -> Result<(), Error> {
         self.urlencoder.append_pair(self.key, value);
         Ok(())