Make body argument optional on #create_issue and #create_pull_request

## Background
GitHub API v3 has been adding more validations to the API. Recently, the validation logic for issue and pull request body was changed so that it doesn't allow `body` parameter with `null` value. For this API change, `create_issue(repo, title, nil)` and `create_pull_request(repo, base, head, title, nil)` begun to raise 422 error.

FYI: maybe related to https://github.com/octokit/octokit.rb/pull/538

## Changes
To fix this problem, I added the following changes:

1. Pass a string value for the body, or no value at all
2. Make body argument optional