JSON file with same name as JS file is fingerprinted in code but not on disk, if 'json' not in extensions to process

Repro steps:

```
npm install --save broccoli
npm install --save broccoli-asset-rev
mkdir app
echo "hello" > app/test.js
echo "there" > app.test.json
```

Create `app/index.html` containing

```

<script src="test.json">
```

Create Brocfile.js containing:

```
var AssetRev = require('broccoli-asset-rev');
var assetNode = new AssetRev('app', {});
module.exports = assetNode;
```

Run `broccoli build dist`

Now, look in the 'dist' folder. It contains

``` index.html
test-b1946ac92492d2347c6235b4d2611184.js
test.json
```

so you can see that test.js has been fingerprinted, and test.json has not. This is reasonable, the default file extensions for broccoli-asset-rev contains "js" but not "json".

Now look inside `dist/index.html`. It contains:

```
<script src="test-b1946ac92492d2347c6235b4d2611184.js">
<script src="test-b1946ac92492d2347c6235b4d2611184.json">
```

Notice that it has replaced the reference to test.json with the same fingerprint that it used for test.js, even though a:test.json should have a different fingerprint as it has different contents, and b:the test.json file in the 'dist' folder didn't get renamed, so the reference shouldn't have changed.

If I add 'json' to the list of extensions to process in the options, then it starts to work properly.