Handle non-iterable arguments reasonable

cryptography/fernet.py

@@ -131,6 +131,7 @@ class Fernet(object):
 
 class MultiFernet(object):
     def __init__(self, fernets):
+        fernets = list(fernets)
         if not fernets:
             raise ValueError(
                 "MultiFernet requires at least one Fernet instance"

tests/test_fernet.py

@@ -146,3 +146,7 @@ class TestMultiFernet(object):
     def test_no_fernets(self, backend):
         with pytest.raises(ValueError):
             MultiFernet([])
+
+    def test_non_iterable_argument(self, backend):
+        with pytest.raises(TypeError):
+            MultiFernet(None)