fix: Error: socket hang up

The fix proposed in probot#122

Author: zregvart

index.js

@@ -27,6 +27,7 @@ class Client {
     const req = superagent.post(target).send(data.body)
 
     delete data.body
+    delete data['content-length']
 
     Object.keys(data).forEach(key => {
       req.set(key, data[key])