Fix the issue when docker exec a paused container, it will always hang.
Add the test case of this issue.

Docker-DCO-1.1-Signed-off-by: Chen Chao <cc272309126@gmail.com> (github: cc272309126)
Merge pull request #9356 from cc272309126/fix-exec-paused-container

Fix the issue that when docker exec a paused container, it will always