Update message method so that its return value is Deref<&str>

src/error.rs

@@ -100,10 +100,8 @@ impl StructuredError {
   /// reporting mechanism. Instead of calling this method, you can access the 
   /// StructuredError `message` field directly.
   #[deprecated(since="0.3.3", note="Please use the `message` field directly instead.")]
-  pub fn message(&self) -> String {
-    self.message
-      .clone()
-      .unwrap_or("".to_string())
+  pub fn message(&self) -> &String {
+    self.message.as_ref().unwrap()
   }
 
   /// Returns the provided c_str as Some(String), or None if the provided pointer is null.