fix: Don't mutate passed URL object (#470)

Before this change, a passed URL object would be mutated if query
parameters were appended via the `data` option.

Now we clone the object instead, so the original is not affected.

---

Background:

I am passing `URL` objects to the `request` method, and then adding
query parameters via the `data` option. Example:

```js
import { request } from "urllib";

const url = new URL("http://example.com");

await request(url, { data: { param1: "value1" } });
```

I found that if I then make another request to the same URL but with
different parameters:

```js
await request(url, { data: { param2: "value2" } });
```

...a request is actually made to
`http://example.com?param1=value1&param2=value2`.

My URL object is being mutated by the urllib library. I see this as a
bug.

src/HttpClient.ts

@@ -201,7 +201,8 @@ export class HttpClient extends EventEmitter {
         // url maybe url.parse(url) object in urllib2
         requestUrl = new URL(urlFormat(url));
       } else {
-        requestUrl = url;
+        // or even if not, we clone to avoid mutating it
+        requestUrl = new URL(url.toString());
       }
     }
 

test/options.data.test.ts

@@ -43,6 +43,15 @@ describe('options.data.test.ts', () => {
     assert.equal(url.searchParams.get('data'), '哈哈');
   });
 
+  it('should not mutate a passed URL object when setting query string', async () => {
+    const url = new URL(_url);
+    assert.equal(url.searchParams.get('param1'), null);
+    await urllib.request(url, {
+      data: { param1: 'val1' },
+    });
+    assert.equal(url.searchParams.get('param1'), null);
+  });
+
   it('should GET with data work on nestedQuerystring=true', async () => {
     const response = await urllib.request(_url, {
       method: 'GET',