[webview_flutter_wkwebview] Fix bug of overriding default values of NSURLRequest (flutter#5969)

Author: bparrishMines

packages/webview_flutter/webview_flutter_wkwebview/example/ios/RunnerTests/FWFDataConvertersTests.m

@@ -25,6 +25,16 @@ - (void)testFWFNSURLRequestFromRequestData {
   XCTAssertEqualObjects(request.allHTTPHeaderFields, @{@"a" : @"header"});
 }
 
+- (void)testFWFNSURLRequestFromRequestDataDoesNotOverrideDefaultValuesWithNull {
+  NSURLRequest *request =
+      FWFNSURLRequestFromRequestData([FWFNSUrlRequestData makeWithUrl:@"https://flutter.dev"
+                                                           httpMethod:nil
+                                                             httpBody:nil
+                                                  allHttpHeaderFields:@{}]);
+
+  XCTAssertEqualObjects(request.HTTPMethod, @"GET");
+}
+
 - (void)testFWFNSHTTPCookieFromCookieData {
   NSHTTPCookie *cookie = FWFNSHTTPCookieFromCookieData([FWFNSHttpCookieData
       makeWithPropertyKeys:@[ [FWFNSHttpCookiePropertyKeyEnumData

packages/webview_flutter/webview_flutter_wkwebview/ios/Classes/FWFDataConverters.m

@@ -17,8 +17,12 @@
     return nil;
   }
 
-  [request setHTTPMethod:data.httpMethod];
-  [request setHTTPBody:data.httpBody.data];
+  if (data.httpMethod) {
+    [request setHTTPMethod:data.httpMethod];
+  }
+  if (data.httpBody) {
+    [request setHTTPBody:data.httpBody.data];
+  }
   [request setAllHTTPHeaderFields:data.allHttpHeaderFields];
 
   return request;