fix: merge existing params in url, close encode#3433

Author: GreyElaina

httpx/_models.py

@@ -396,7 +396,9 @@ def __init__(
         extensions: RequestExtensions | None = None,
     ) -> None:
         self.method = method.upper()
-        self.url = URL(url) if params is None else URL(url, params=params)
+        self.url = URL(url)
+        if params is not None:
+            self.url = self.url.copy_merge_params(params=params)
         self.headers = Headers(headers)
         self.extensions = {} if extensions is None else dict(extensions)