refactor(redirect): Use "master" for default branch

It redirects to the default branch not head hash.

src/libs/redirect.ts

@@ -7,7 +7,7 @@ import { getRepository } from "./env.ts";
 export function redirect<R extends string>(
   ctx: RouterContext<R>,
   userAgent: UserAgent,
-  ref: string = "HEAD"
+  ref: string = "master"
 ): void {
   const repository = getRepository();
   const url = join(

test/libs/redirect_test.ts

@@ -13,7 +13,7 @@ Deno.test("Redirect Detection", <R extends string>() => {
   assertEquals(ctx.response.status, STATUS_CODE.PermanentRedirect);
   assertEquals(
     ctx.response.headers.get("location"),
-    `https://github.com/${testRepo.owner}/${testRepo.name}/blob/HEAD/${testRepo.path}`
+    `https://github.com/${testRepo.owner}/${testRepo.name}/blob/master/${testRepo.path}`
   );
 });