Return EAGAIN for zero-length nonblocking reads
Return EAGAIN for zero-length nonblocking reads

At some point since my framing loop patch, the filter pipeline started
returning APR_SUCCESS instead of APR_EAGAIN for a nonblocking read of
zero bytes. This caused the work_done flag to always be set, which meant
the framing loop never paused to wait for incoming data, and the module
would rail the CPU.

mod_websocket_read_nonblock() now checks for this situation and converts
the status back to APR_EAGAIN.

jchampio/apache-websocket,jchampio/apache-websocket,jchampio/apache-websocket