Fix bytes determination bug [BR#289: youfou]

.gitignore

 build/*
 dist/*
-test/*
+tests/*
 itchat.egg-info/*
 *.pyc
 *.swp

itchat/config.py

 import os, platform
 
-VERSION = '1.2.31'
+VERSION = '1.2.32'
 BASE_URL = 'https://login.weixin.qq.com'
 OS = platform.system() #Windows, Linux, Darwin
 DIR = os.getcwd()

itchat/utils.py

@@ -134,10 +134,10 @@ def contact_deep_copy(core, contact):
 
 def get_image_postfix(data):
     data = data[:20]
-    if 'GIF' in data:
+    if b'GIF' in data:
         return 'gif'
-    elif 'PNG' in data:
+    elif b'PNG' in data:
         return 'png'
-    elif 'JFIF' in data:
+    elif b'JFIF' in data:
         return 'jpg'
     return ''