Correcting HttpContentDecoder 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