[hbase] Allow to empty a cell

Api Error: putColumn() takes exactly 6 arguments (5 given)

In Hbase app, this error occur in three situations:

1. Open full editor in any cell, remove all the content (empty cell value). Click save. Error occurs.
2. Click Create New Column in any row, leave the cell value blank. Click submit. Error occurs.
3. Create an empty cell using the new row button. Upload pdf to the empty cell, click save. Error occurs.

An correct Put request in log: "POST /hbase/api/putColumn/Cluster/document_demo/20150722/%22doc%3Atxt%22/hbase-post-key-0 HTTP/1.1"
The one that incur the error:
"POST /hbase/api/putColumn/%22Cluster%22/%22document_demo%22/20150722/%22doc%3Apdf%22 HTTP/1.1”

Full error log:

[22/Jul/2015 12:05:23 -0700] middleware INFO Processing exception: Api Error: putColumn() takes exactly 6 arguments (5 given): Traceback (most recent call last):
File "/home/yixiao/workspace/master_hue/hue/build/env/lib/python2.6/site-packages/Django-1.6.10-py2.6.egg/django/core/handlers/base.py", line 112, in get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/home/yixiao/workspace/master_hue/hue/build/env/lib/python2.6/site-packages/Django-1.6.10-py2.6.egg/django/db/transaction.py", line 371, in inner
return func(*args, **kwargs)
File "/home/yixiao/workspace/master_hue/hue/apps/hbase/src/hbase/views.py", line 77, in api_router
return api_dump(HbaseApi(request.user).query(*url_params))
File "/home/yixiao/workspace/master_hue/hue/apps/hbase/src/hbase/api.py", line 54, in query
raise PopupException(_("Api Error: %s") % e.message)
PopupException: Api Error: putColumn() takes exactly 6 arguments (5 given)