fix(common): AuthenticationType implements Str

Instead of an arbitrary `url()` method with the signature of
`to_slice()`

src/common.rs

@@ -23,9 +23,9 @@ pub enum AuthenticationType {
     Device,
 }
 
-impl AuthenticationType {
+impl Str for AuthenticationType {
     /// Converts itself into a URL string
-    pub fn url(&self) -> &'static str {
+    fn as_slice(&self) -> &'static str {
         match *self {
             AuthenticationType::Device => "https://accounts.google.com/o/oauth2/device/code",
         }

src/device.rs

@@ -150,7 +150,7 @@ impl<NC> DeviceFlow<NC>
                                    .collect::<String>()
                                    .as_slice())].iter().cloned());
 
-        match self.client.post(AuthenticationType::Device.url())
+        match self.client.post(AuthenticationType::Device.as_slice())
                .header(ContentType("application/x-www-form-urlencoded".parse().unwrap()))
                .body(req.as_slice())
                .send() {

src/refresh.rs

@@ -68,7 +68,7 @@ impl<NC> RefreshFlow<NC>
                                 .iter().cloned());
 
         let json_str = 
-            match self.client.post(auth_type.url())
+            match self.client.post(auth_type.as_slice())
                .header(ContentType("application/x-www-form-urlencoded".parse().unwrap()))
                .body(req.as_slice())
                .send() {