check for None value in utils.safe_header

In case that a sensetive header (that should be obscured by its SHA1
hash) is None, the safe_header throws an exception which fails the
calling process and by that may harm the functionality.

Change-Id: I56944a382fd546eba0a6dd6d6b1cecf83b1dc106
Closes-Bug: #1491311

glanceclient/common/utils.py

@@ -450,7 +450,7 @@ def memoized_property(fn):
 
 
 def safe_header(name, value):
-    if name in SENSITIVE_HEADERS:
+    if value is not None and name in SENSITIVE_HEADERS:
         v = value.encode('utf-8')
         h = hashlib.sha1(v)
         d = h.hexdigest()