Fix Log Warning on Shutdown
Fix Log Warning on Shutdown

Upon shutdown, this error was displayed:
Exception AttributeError: "'NoneType' object has no attribute 'debug'" in <bound method Connection.__del__ of <happybase.api.Connection object at 0x106878250>> ignored

TAKEALOT/happybase,georgesuperman/happybase,wfxiang08/happybase,tekton/happybase,rickysaltzer/happybase