Preserve an explicit Content-Type returned through github.Response

[tylergannon]

Problem

In v0.2.0, writeResults copies headers from the service's *github.Response, then unconditionally replaces Content-Type with application/json for JSON bodies.

That contradicts the documented behavior that a returned response can set explicit headers and prevents a GitHub-compatible implementation from returning application/json; charset=utf-8.

Reproduction

Return a value plus:

&github.Response{Response: &http.Response{
    StatusCode: http.StatusCreated,
    Header: http.Header{
        "Content-Type": {"application/json; charset=utf-8"},
    },
}}

A real go-github client observes Content-Type: application/json.

Requested behavior

Set application/json only when no Content-Type has already been supplied. Add a real go-github round-trip test proving an explicit Content-Type survives.