Not able to get the ADLS File System Object - Throws exception

### Description
When I try to create the `core.AzureDLFileSystem` Object I am getting following exception.

```
File "", line 1, in
File "/usr/local/lib/python3.7/site-packages/azure/datalake/store/core.py", line 72, in __init__
self.connect()
File "/usr/local/lib/python3.7/site-packages/azure/datalake/store/core.py", line 90, in connect
self.azure = DatalakeRESTInterface(token=self.token, req_timeout_s=self.per_call_timeout_seconds, **self.kwargs)
File "/usr/local/lib/python3.7/site-packages/azure/datalake/store/lib.py", line 273, in __init__
self._check_token() # Retryable method. Will ensure that signed_session token is current when we set it on next line
File "/usr/local/lib/python3.7/site-packages/azure/datalake/store/lib.py", line 307, in _check_token
check_token_internal()
File "/usr/local/lib/python3.7/site-packages/azure/datalake/store/retry.py", line 93, in f_retry
out = func(*args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/azure/datalake/store/lib.py", line 303, in check_token_internal
cur_session = self.token.signed_session()
AttributeError: 'str' object has no attribute 'signed_session'
```

### Reproduction Steps

tannent_id, username and password has changed.

```python
from azure.datalake.store import lib,core
tenant_id="12b02d9b-d7f2-204e-b345-0dbdcf0330f8"
token = lib.auth(tenant_id, "gaurang.shah@abc.com", "MyPassword")
adl = core.AzureDLFileSystem('myadlsfilesystem', token)
```

### Environment summary

**SDK Version:** What version of the SDK are you using? (pip show azure-datalake-store)
Answer here:
Name: azure-datalake-store
Version: 0.0.47
Summary: Azure Data Lake Store Filesystem Client Library for Python
Home-page: https://github.com/Azure/azure-data-lake-store-python
Author: Microsoft Corporation
Author-email: ptvshelp@microsoft.com
License: MIT License
Location: /usr/local/lib/python3.7/site-packages
Requires: adal, cffi, requests
Required-by:

**Python Version:** What Python version are you using? Is it 64-bit or 32-bit?
Answer here:
64 bit Python 3.7.4 for mac

Python 3.7.4 (default, Jul 9 2019, 18:13:23)
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin

**OS Version:** What OS and version are you using?
Answer here: Mac 10.14.6

**Shell Type:** What shell are you using? (e.g. bash, cmd.exe, Bash on Windows)
Answer here: Bash

Riproduci la chiamata al costruttore segnalata con azure.datalake.store.lib.auth e core.AzureDLFileSystem utilizzando le versioni indicate dell’SDK e di Python. Inizia da core.py:72 e lib.py:273-307, quindi segui il token passato a DatalakeRESTInterface; il lavoro è completato quando è possibile creare l’oggetto del filesystem senza l’AttributeError segnalato.

Scritto dal modello di indicizzazione a partire dal testo della issue.