Don't raise TypeError for invalid b64

In Python 2, the 'b64decode' function calls the 'binascii.a2b_base64'
function but catches any 'binascii.Error' exceptions raised and raises a
TypeError instead [1]. In Python 3, a 'binascii.Error' error is raised
instead [2]. Rather than forcing users to handle two types of exception,
we can allow them to catch only the 'bisascii.Error'. Python 2 provides
a function that does just this - 'decodestring' - which we can use. Make
it so.

[1] https://github.com/python/cpython/blob/2.7/Lib/base64.py#L78
[2] https://github.com/python/cpython/blob/3.5/Lib/base64.py#L87
[3] https://github.com/python/cpython/blob/2.7/Lib/base64.py#L326

Change-Id: I72c6de71b174181292427128d20e03756f85fb97

oslo_serialization/base64.py

@@ -67,7 +67,10 @@ def decode_as_bytes(encoded):
     """
     if isinstance(encoded, bytes):
         encoded = encoded.decode('ascii')
-    return base64.b64decode(encoded)
+    if six.PY2:
+        return base64.decodestring(encoded)
+    else:
+        return base64.b64decode(encoded)
 
 
 def decode_as_text(encoded, encoding='utf-8'):

oslo_serialization/tests/test_base64.py

@@ -13,6 +13,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import binascii
+
 from oslo_serialization import base64
 from oslotest import base as test_base
 
@@ -45,6 +47,11 @@ class Base64Tests(test_base.BaseTestCase):
         self.assertEqual(b'text',
                          base64.decode_as_bytes(u'dGV4dA=='))
 
+    def test_decode_as_bytes__error(self):
+        self.assertRaises(binascii.Error,
+                          base64.decode_as_bytes,
+                          'hello world')
+
     def test_decode_as_text(self):
         self.assertEqual(u'text',
                          base64.decode_as_text(b'dGV4dA=='))