Major Description
3 changes I think should be made for FlushResultHandler:
1) I forgot about FlushHeadersOutputStream should override OutputStream.close().
2) The FlushHeadersOutputStream's flushHeaders should be called directly after the MessageBodyWriter.writeTo is called and the DataContentHandler.writeTo. The MessageBodyWriter is not guaranteed to write anything to the output stream but could just be pushing headers out (i.e. some headers to real content).
3) The logger.warn() and logger.error() should be turned into an Info. I don't think there's any reason to do repeated warnings to an administrator.