Static HTML generator drops response headers

``` json
"responses" : {
  "500" : {
    "description" : "Ouch",
    "headers" : {
      "X-Request-ID" : {
        "type" : "string",
        "description" : "Blah"
      }
    }
  }
}
```

This is rendered in the editor, but missing from the static HTML.

`swagger-codegen-cli:2.1.5`
