feat: use baseUrl in RequestOptions as 1st priority

add smaple code and unit test

fix cfug#385

example/options.dart

@@ -40,4 +40,8 @@ main() async {
         contentType: ContentType.parse("application/x-www-form-urlencoded")),
   );
   print(response.data);
+
+  response = await dio.request("/get",
+      options: RequestOptions(baseUrl: "http://httpbin.org/"));
+  print(response.data);
 }

package_src/lib/src/dio.dart

@@ -947,10 +947,11 @@ class Dio {
       Options opt, String url, data, Map<String, dynamic> queryParameters) {
     var query = (Map<String, dynamic>.from(options.queryParameters ?? {}))
       ..addAll(queryParameters ?? {});
+    final optBaseUrl = (opt is RequestOptions) ? opt.baseUrl: null;
     return RequestOptions(
       method: (opt.method ?? options.method)?.toUpperCase() ?? "GET",
       headers: (Map.from(options.headers))..addAll(opt.headers),
-      baseUrl: options.baseUrl ?? "",
+      baseUrl: optBaseUrl ?? options.baseUrl ?? "",
       path: url,
       data: data,
       connectTimeout: opt.connectTimeout ?? options.connectTimeout ?? 0,

package_src/test/dio_test.dart

@@ -374,4 +374,23 @@ void main() {
       ]);
     });
   });
+
+  group("basic tests", () {
+    Dio dio;
+    setUp(() {
+      dio = Dio();
+      // dio.options.baseUrl = MockAdapter.mockBase;
+      dio.options.headers = {'User-Agent': 'dartisan', 'XX': '8'};
+      dio.httpClientAdapter = MockAdapter();
+    });
+    test("base url in request options", () async {
+      Response response;
+      response = await dio.get(
+        "/test",
+        queryParameters: {"id": '12', "name": "wendu"},
+        options: RequestOptions(baseUrl: MockAdapter.mockBase),
+      );
+      expect(response.data["errCode"], 0);
+    });
+  });
 }