Preserve Exif Data

# Expected Behavior

Image is being saved with a horizontal and vertical resolution of 96 dpi, even though the input image is 1 dpi. The Resolution Unit also doesn't appear to be saved.

# Current Behavior

Jimp is setting its own values, regardless of the input image. There doesn't appear to be a way to change these settings either.

# Failure Information (for bugs)

I would expect the meta data to remain the same when saving an image from an input image, but it doesn't appear so.

## Steps to Reproduce

```
var jimp = require('jimp');
const fs = require('fs');

jimp.read('image.jpg', (err, img1) => {
if (err) throw err;
const debugOut = JSON.parse(JSON.stringify(img1));
debugOut.bitmap.data = [];
fs.writeFile('data.txt', JSON.stringify(debugOut, null, 4), (err3) => {
if (err3) throw err;
});
img1.write('OUTPUT.JPG');
});
```

## Context

I'm working with equirectangular images and creating a script to stitch images together from a Yi VR camera. It is paramount that the metadata stays intact for the images otherwise they can't be stitched. Images can be manually edited in paint, but I it would go much faster if I can automate it. Jimp seems to be ignoring the metadata for an image, no matter what I set it to:
```
img1._exif.XResolution = 1;
img1._exif.YResolution = 1;
```

- Jimp Version: 0.3.5
- Operating System: Windows 10
- Node version: 8.9.4