{bool allowMalformed: false}
)
Instantiates a new Utf8Codec.
The optional allowMalformed argument defines how decoder (and decode)
deal with invalid or unterminated character sequences.
If it is true (and not overriden at the method invocation) decode and
the decoder replace invalid (or unterminated) octet
sequences with the Unicode Replacement character U+FFFD (�). Otherwise
they throw a FormatException.