Fix #78 again.

Author: Eric-Guo

lib/wechat/http_client.rb

@@ -22,7 +22,7 @@ def get(path, get_header = {})
     def post(path, payload, post_header = {})
       request(path, post_header) do |url, header|
         params = header.delete(:params)
-        httprb.headers(header).post(url, params: params, body: payload, ssl_context: ssl_context)
+        httprb.headers(header).post(url, params: params, body: payload.to_json, ssl_context: ssl_context)
       end
     end