Motivation:
Other implementations of FullHttpMessage allow .toString to be called after the Message has been released
This brings AggregatedFullHttpMessage into line with those impls.
Modifications:
- Changed AggregatedFullHttpMessage to no longer be a sub-class of DefaultByteBufHolder
- Changes AggregatedFullHttpMessage to implement ByteBufHolder
- Hold the content buffer internally to AggregatedFullHttpMessage
- Implement the required content() and release() methods that were missing
- Do not check refcnt when accessing content() (similar to DefaultFullHttpMessage)
Result:
A released AggregatedFullHttpMessage can have .toString called without throwing an exception
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