Fix setting identifier truncation in SettingsFrame.serialize_body

The bitmask `setting & 0xFF` truncates the 16-bit setting identifier
to only 8 bits. This silently corrupts any setting ID above 255 during
serialization.

The identifier field in SETTINGS frames is 16 bits wide (RFC 9113,
Section 6.5.1), and `_STRUCT_HL` already uses the `H` format
(unsigned short, 16 bits) for it. However, `& 0xFF` discards the
upper byte.

For the standard settings (0x01 through 0x08) this has no visible
effect since they all fit in 8 bits. But RFC 8701 GREASE values like
0x0a0a or 0x1a1a, as well as any future extension settings > 0xFF,
would be silently mangled on a serialize round-trip.

Change the mask to `& 0xFFFF` to match the actual field width.

Author: bysiber

src/hyperframe/frame.py

@@ -457,7 +457,7 @@ def _body_repr(self) -> str:
         return f"settings={self.settings}"
 
     def serialize_body(self) -> bytes:
-        return b"".join([_STRUCT_HL.pack(setting & 0xFF, value)
+        return b"".join([_STRUCT_HL.pack(setting & 0xFFFF, value)
                          for setting, value in self.settings.items()])
 
     def parse_body(self, data: memoryview) -> None: