Handle explicitly passed None value

tomquirk#128 adds the ability to customize cookie dir. But
it also allows passing `None` as an argument for `cookies_dir`, which
raises `TypeError` while calling `os.path.exists(self.cookie_dir)`.

Setting `cookies_dir`'s default value to `None` and handling `None`
while setting it to the `self` fixes this unexpected behavior.

linkedin_api/cookie_repository.py

@@ -21,8 +21,8 @@ class CookieRepository(object):
     TODO: refactor to use http.cookiejar.FileCookieJar
     """
 
-    def __init__(self, cookies_dir=settings.COOKIE_PATH):
-        self.cookies_dir = cookies_dir
+    def __init__(self, cookies_dir=None):
+        self.cookies_dir = cookies_dir if cookies_dir else settings.COOKIE_PATH
 
     def save(self, cookies, username):
         self._ensure_cookies_dir()