def METHOD_NAME(self, mock_post):
    # Create response mock
    mock_resp = MagicMock()
    mock_resp.status_code = 201
    mock_post.return_value = mock_resp
    gh = GithubHelper('access_token', 'username')
    status_code = gh.create_pull_request('owner', 'repo', 'head', 'base', {'header': 'value'})
    self.assertEqual(status_code, 201)
    mock_post.assert_called_once_with(
        'https://api.github.com/repos/owner/repo/pulls',
        json={
            'title': 'Pull request by username',
            'body': 'Please review and merge this change.',
            'head': 'username:head',
            'head_repo': 'repo',
            'base': 'base'
        },
        headers={'header': 'value'}
    )