Avoid missing content-length header in project.py

Occassionally, the content-length may be missing when using urlib
in python 2.6 and 2.7.  This change assumes the value of the header is
0 if it doesn't exist

Change-Id: Iaf1c8a796bc667823d4d7c30f9b617644b271d00

project.py

@@ -1529,7 +1529,7 @@ class Project(object):
 
       p = None
       try:
-        size = r.headers['content-length']
+        size = r.headers.get('content-length', 0)
         unit = 1 << 10
 
         if size and not quiet: