add integration test case

AmplifyPlugins/API/Sources/AWSAPIPlugin/Operation/AWSRESTOperation.swift

@@ -116,7 +116,7 @@ final public class AWSRESTOperation: AmplifyOperation<
                 }
             }
 
-            // apply customized request headers
+            // The headers from the request object should override any of the same header modifications done by the intercepters above
             finalRequest = RESTOperationRequestUtils.applyCustomizeRequestHeaders(request.headers, on: finalRequest)
 
             if isCancelled {

AmplifyPlugins/API/Tests/APIHostApp/AWSAPIPluginRESTIAMTests/RESTWithIAMIntegrationTests.swift

@@ -159,6 +159,25 @@ class RESTWithIAMIntegrationTests: XCTestCase {
             XCTAssertEqual(statusCode, 404)
         }
     }
+
+    func testRestRequest_withCustomizeHeaders_succefullyOverride() async throws {
+        let request = RESTRequest(path: "/items", headers: ["Content-Type": "text/plain"])
+        do {
+            _ = try await Amplify.API.get(request: request)
+            XCTFail("Should catch error")
+        } catch {
+            guard let apiError = error as? APIError else {
+                XCTFail("Error should be APIError")
+                return
+            }
+            guard case let .httpStatusError(statusCode, _) = apiError else {
+                XCTFail("Error should be httpStatusError")
+                return
+            }
+
+            XCTAssertEqual(statusCode, 403)
+        }
+    }
 }
 
 extension RESTWithIAMIntegrationTests: DefaultLogger { }