Fix can not automerge when description is too long

When MR description is too long (ex. over 4000 characters) , can not accept.

Because 414 Error (Request-URI Too Large) may occur depending on the configuration of the server.

![mr1](https://cloud.githubusercontent.com/assets/608755/2744357/aeb33d62-c724-11e3-8c63-81c58a75c7b3.png)

![mr](https://cloud.githubusercontent.com/assets/608755/2744359/afc1e12c-c724-11e3-812d-105eebc314da.png)