Correcting HttpDecoder @SuppressWarnings parameter

Motiviation:
The HttpContentDecoder.getTargetContentEncoding has a @SuppressWarnings(unused) on its parameter.
This should be @SuppressWarnings(UnusedParameters).

Modifications:
@SuppressWarnings(unused) -> @SuppressWarnings(UnusedParameters)

Result:
Correctly suppressing warnings due to HttpContentDecoder.getTargetContentEncoding