canvas_api.py: Fix pagination with 'link' -> 'Link'

canvas_api.py

@@ -27,7 +27,7 @@ class Canvas(object):
             # check for 'Link' header
             linkHdr = None
             for h in resp.headers.items():
-                if h[0] == 'link':
+                if h[0] == 'Link':
                     linkHdr = h
                     break
             else: