moved open_key check upper

Author: eegemberdiev

open_api.py

@@ -3,8 +3,8 @@
 
 class OpenPlatform(object):
     def __init__(self, open_key=''):
-        self.headers = {'Authorization': open_key}
         if open_key == '':
             raise AttributeError('open_key can not be empty')
+        self.headers = {'Authorization': open_key}
         self.scaffold = Scaffold(self.headers)
         self.shareholder = Shareholder(self.headers)