HttpResponseException.getContent() should return an InputStream

#### Environment details

- OS: Win 10
- Java version: 8 u144
- google-http-java-client version: 1.25.0

#### Steps to reproduce

1. Create a request for the URL: `https://maps.googleapis.com/maps/api/staticmap?markers=-30.93963400,-61.56805900&size=672x297`
2. It will throw a `HttpResponseException` since the page returns a `403 Forbidden`

The problem here is that `HttpResponseException.getContent()` returns a `String` (implemented for #143). However, the response for that URL is an image (`content-type: image/png`).

I think that `HttpResponseException.getContent()` should be equal to `HttpResponse.getContent()`. It is currently being parsed at [HttpResponseException.java#L175](https://github.com/googleapis/google-http-java-client/blob/master/google-http-client/src/main/java/com/google/api/client/http/HttpResponseException.java#L175).