url_preview: Allow Beautiful Soup to get the charset from <meta>.

An HTML document sent without a charset in the Content-Type header
needs to be scanned for a charset in <meta> tags.  We need to pass
bytes instead of str to Beautiful Soup to allow it to do this.

Fixes #16843.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/test_helpers.py

@@ -314,6 +314,7 @@ class HostRequestMock:
 
 class MockPythonResponse:
     def __init__(self, text: str, status_code: int, headers: Optional[Dict[str, str]]=None) -> None:
+        self.content = text.encode()
         self.text = text
         self.status_code = status_code
         if headers is None: