Don't use CowStrDeserializer (fixes #53)

src/de.rs

@@ -194,7 +194,10 @@ impl<'de> de::Deserializer<'de> for Part<'de> {
     where
         V: de::Visitor<'de>,
     {
-        self.0.into_deserializer().deserialize_any(visitor)
+        match self.0 {
+            Cow::Borrowed(value) => visitor.visit_borrowed_str(value),
+            Cow::Owned(value) => visitor.visit_string(value),
+        }
     }
 
     fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>

tests/test_deserialize.rs

@@ -29,6 +29,13 @@ fn deserialize_str() {
     assert_eq!(serde_urlencoded::from_str("first=23&last=42"), Ok(result));
 }
 
+#[test]
+fn deserialize_borrowed_str() {
+    let result = vec![("first", 23), ("last", 42)];
+
+    assert_eq!(serde_urlencoded::from_str("first=23&last=42"), Ok(result));
+}
+
 #[test]
 fn deserialize_reader() {
     let result = vec![("first".to_owned(), 23), ("last".to_owned(), 42)];