Fixed #1711
added test case related to #1711
added new method 'merge_cookies' and use it when merging session cookies with param cookies.
update 'merge_cookies'
modified 'merge_cookies' and update tests.
made sessions.py to use that method when merging cookies.
update 'merge_cookies' not to create new instance.
double try statement
remove unnecessary try
remove unused import
Merge pull request #1713 from daftshady/master

Fixed #1711