Refactor unpinPost() to use template request

Sources/WriteFreely/WFClient.swift

@@ -389,29 +389,16 @@ public class WFClient {
             completion(.failure(error))
         }
 
-        let dataTask = session.dataTask(with: request) { (data, response, error) in
-            // Something went wrong; return the error message.
-            if let error = error {
-                completion(.failure(error))
-            }
-
-            if let response = response as? HTTPURLResponse {
-                guard let data = data else { return }
-
-                // If we get a 200 OK, return the WFUser as success; if not, return a WFError as failure.
-                if response.statusCode == 200 {
+        post(with: request, expecting: 200) { result in
+            switch result {
+            case .success(_):
                 completion(.success(true))
-                } else {
-                    // We didn't get a 200 OK, so return a WFError
-                    guard let error = self.translateWFError(fromServerResponse: data) else { return }
+            case .failure(let error):
                 completion(.failure(error))
             }
         }
     }
 
-        dataTask.resume()
-    }
-
     /// Creates a new post.
     ///
     /// Creates a new post. If a `collectionAlias` is provided, the post is published to that collection; otherwise, it