Fix :OpenGithubPullReq when it has no argument

Previously, it opens '/issues' page when it has no argument because
:OpenGithubPullReq is the same as :OpenGithubIssues.
This commit fix the issue by making :OpenGithubPullReq open '/pulls'
page when it has no argument.
fix :OpenGithubPullReq when PR number is specified
Merge pull request #11 from rhysd/fix/OpenGithubPullReq

Fix :OpenGithubPullReq when it has no argument