Fix csrf validation without time limit (time_limit=False)

With time_limit=False, generate_csrf will generate a token with an empty
'expires' value. The validate_csrf function will always fail because it
tries to parse an empty string as float. We should parse this value only
if we care about the time_limit.
Merge pull request #129 from BenoitZugmeyer/non-expiring-csrf

Fix csrf validation without time limit (time_limit=False)