BUG/MEDIUM: http-client: Only consume input buffer when hc one is empty

BUG/MEDIUM: http-client: Only consume input buffer when hc one is empty

Since http-client applet uses its own buffers, it is possible to have data
stuck in the applet input buffer while the http-client response buffer is
full, preventing the applet to consume these data. If this happens on the
last part of the response payload, the upper stream can decide to shut the
applet. In this case, the applet using the http client will not be able to
retrieve these last data because they will never be move into the hc
response buffer.

The main reason for this bug is that, for now, the applets cannot survive
the upper stream unlike multiplexers. It could be a good improvement for the
3.5. However, some applets still uses the stream-connector and the upper
stream (peer and stat applets for instance). So it is not an easy task.

In the mean time, to fix the issue on stable branches, the http-client
applet now stops to consume data when the hc response buffer is not empty.
This way, the applet shut will be deferred. Data will be consumed when they
can be fully moved in the httpclient response buffer.

This patch should fix the issue #3366. It must be backported to 3.3.