Account.get_tags_used_by_author fails on appbase

Expected behavior

Account.get_tags_used_by_author should return a list of tags or an empty list

Actual behavior

Account.get_tags_used_by_author fails with an RPC error on appbase nodes:

>>> a.get_tags_used_by_author()
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/beemapi/steemnoderpc.py", line 64, in rpcexec
    reply = super(SteemNodeRPC, self).rpcexec(payload)
  File "/usr/local/lib/python3.6/site-packages/beemapi/graphenerpc.py", line 409, in rpcexec
    raise RPCError(ret['error']['message'])
beemapi.exceptions.RPCError: Assert Exception:acnt != nullptr: Author not found

How to reproduce

from beem.account import Account
from beem import Steem
s = Steem(node=['https://api.steemit.com'])
a = Account('crokkon', steem_instance=s)
print(a.get_tags_used_by_author())

get_tags_used_by_author requires an argument author, but currently gets the argument account:

beem/beem/account.py

Line 1144 in 243bb99

    
           return self.steem.rpc.get_tags_used_by_author({'account': account}, api="tags")['tags']

Environment

# beempy --version
beempy, version 0.19.50
# python --version
Python 3.6.6