Fix WeChatMessage.send_markdown (#450)
Fix WeChatMessage.send_markdown (#450)

1. replace discarded method with `send`,  [ref commit](https://github.com/jxtech/wechatpy/commit/c876a71c5f21f3a5449abed72a5656b5e5c1b13c#diff-8d5d79095778bde1f0cd139596ba9d90)
2. unify params used as other methods
jxtech/wechatpy,wechatpy/wechatpy,messense/wechatpy