Accepts parameter for Patch

This is a bit of a weird pull request, the app I'm building has Markdown support, so I'm using the underlying ApiConnection class to make use of Octokit, but with a custom accepts header. For instance:

``` csharp
public static Task<FullIssue> CreateFullIssueAsync(this IGitHubClient gitHubClient, string owner, string name, NewIssue issue)
{
    var uri = ApiUrls.Issues(owner, name);

    var apiConnection = new ApiConnection(gitHubClient.Connection);

    return apiConnection.Post<FullIssue>(uri, issue, "application/vnd.github.v3.full+json; charset=utf-8");
}
```

``` csharp
public class FullIssue : Issue
{
    public string BodyText
    {
        get; set;
    }

    public string BodyHtml
    {
        get; set;
    }
}
```

However some of the methods don't have overloads that take an accepts parameter. This will add them.