Automatic merge from submit-queue
Fix hang/websocket timeout when streaming container log with no content
When streaming and following a container log, no response headers are sent from the kubelet `containerLogs` endpoint until the first byte of content is written to the log. This propagates back to the API server, which also will not send response headers until it gets response headers from the kubelet. That includes upgrade headers, which means a websocket connection upgrade is not performed and can time out.
To recreate, create a busybox pod that runs `/bin/sh -c 'sleep 30 && echo foo && sleep 10'`
As soon as the pod starts, query the kubelet API:
```
curl -N -k -v 'https://<node>:10250/containerLogs/<ns>/<pod>/<container>?follow=true&limitBytes=100'
```
or the master API:
```
curl -N -k -v 'http://<master>:8080/api/v1/<ns>/pods/<pod>/log?follow=true&limitBytes=100'
```
In both cases, notice that the response headers are not sent until the first byte of log content is available.
This PR:
* does a 0-byte write prior to handing off to the container runtime stream copy. That commits the response header, even if the subsequent copy blocks waiting for the first byte of content from the log.
* fixes a bug with the "ping" frame sent to websocket streams, which was not respecting the requested protocol (it was sending a binary frame to a websocket that requested a base64 text protocol)
* fixes a bug in the limitwriter, which was not propagating 0-length writes, even before the writer's limit was reached
eeac23282d