Requesting PR causes error if it already exists

[int128]

Steps to reproduce

1. Open some repository page.
2. Click "Request a Pull Request now" button.
3. Click "Request a Pull Request now" button again.

Logs

2019/02/07 04:20:08 DEBUG: 304 GET https://api.github.com/repos/int128/gradle-plugin-starter/contents/gradle/wrapper/gradle-wrapper.properties
2019/02/07 04:20:08 DEBUG: 304 GET https://api.github.com/repos/int128/gradle-plugin-starter/readme
2019/02/07 04:20:08 DEBUG: 304 GET https://services.gradle.org/versions/current
2019/02/07 04:20:10 DEBUG: 304 GET https://api.github.com/repos/int128/gradle-plugin-starter
2019/02/07 04:20:11 DEBUG: 202 POST https://api.github.com/repos/int128/gradle-plugin-starter/forks
2019/02/07 04:20:12 DEBUG: 304 GET https://api.github.com/repos/int128/gradle-plugin-starter/branches/master
2019/02/07 04:20:12 DEBUG: 200 GET https://api.github.com/repos/gradleupdate/gradle-plugin-blank/branches/gradle-5.2-int128
2019/02/07 04:20:13 DEBUG: 200 GET https://api.github.com/repos/int128/gradle-plugin-starter/pulls?base=master&head=gradleupdate%3Agradle-5.2-int128&page=1&per_page=1&state=all
2019/02/07 04:20:14 DEBUG: 422 POST https://api.github.com/repos/int128/gradle-plugin-starter/pulls
2019/02/07 04:20:14 ERROR: error while sending a pull request for int128/gradle-plugin-starter: POST https://api.github.com/repos/int128/gradle-plugin-starter/pulls: 422 Validation Failed [{Resource:PullRequest Field: Code:custom Message:A pull request already exists for gradleupdate:gradle-5.2-int128.}]

Cause

GitHub PullRequest List API https://developer.github.com/v3/pulls/#list-pull-requests does not work properly, maybe due to change of API behavior.