Repository: frictionlessdata/datapackage-py (python)

Issue:
Package(schema='tabular') raises an Exception
To reproduce:

```python
import datapackage
dp = datapackage.Package(schema='tabular')
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
#   File "/home/vitor/Projetos/okfn/frictionlessdata/goodtables-py/env/lib/python3.6/site-packages/datapackage-1.1.4-py3.6.egg/datapackage/package.py", line 52, in __init__
#     descriptor['profile'] = schema
# TypeError: 'NoneType' object does not support item assignment
```

Pull request description:
Fix legacy `tabular` schema Package initiation without a descriptor
- fixes #197 


Patch:
diff --git a/datapackage/package.py b/datapackage/package.py
index 45f3ac0..9f0d558 100644
--- a/datapackage/package.py
+++ b/datapackage/package.py
@@ -63,6 +63,8 @@ class Package(object):
                     schema = 'tabular-data-package'
                 elif schema == 'fiscal':
                     schema = 'fiscal-data-package'
+                if descriptor is None:
+                    descriptor = {}
                 descriptor['profile'] = schema
 
         # Handle deprecated default_base_path argument


Test patch:
diff --git a/tests/test_package.py b/tests/test_package.py
index cdf56b2..334222b 100644
--- a/tests/test_package.py
+++ b/tests/test_package.py
@@ -1337,6 +1337,11 @@ def test_package_save_slugify_fk_resource_name_issue_181():
         {'fields': 'lang', 'reference': {'resource': 'my_langs', 'fields': 'lang'}}
     ]
 
+
+def test_package_legacy_schema_tabular_issue_197():
+    package = Package(None, schema='tabular')
+
+
 # Fixtures
 
 @pytest.fixture