Add missing autoreset in Packer.pack_ext_type (#663)

msgpack/_packer.pyx

@@ -291,6 +291,10 @@ cdef class Packer:
             raise ValueError("ext data too large")
         msgpack_pack_ext(&self.pk, typecode, len(data))
         msgpack_pack_raw_body(&self.pk, data, len(data))
+        if self.autoreset:
+            buf = PyBytes_FromStringAndSize(self.pk.buf, self.pk.length)
+            self.pk.length = 0
+            return buf
 
     @cython.critical_section
     def pack_array_header(self, long long size):

msgpack/fallback.py

@@ -867,6 +867,10 @@ class Packer:
             self._buffer.write(b"\xc9" + struct.pack(">I", L))
         self._buffer.write(struct.pack("B", typecode))
         self._buffer.write(data)
+        if self._autoreset:
+            ret = self._buffer.getvalue()
+            self._buffer = BytesIO()
+            return ret
 
     def _pack_array_header(self, n):
         if n <= 0x0F: