assetMap is rewritten if "json" is in replaceExtensions

Just ran into a tough-to-debug issue with broccoli-asset-rev that was causing issues in FastBoot. FastBoot relies on the asset map to generate its own manifest file.

However, in one of the apps I'm working on, they had added `json` to the `replaceExtensions` array (which I'd guess may be fairly common). Unfortunately, the `assetMap.json` file is also rewritten.

In effect, that means that instead of:

``` js
{ assets:
{ 'assets/module-whitelist.css': 'assets/module-whitelist-d41d8cd98f00b204e9800998ecf8427e.css',
'assets/module-whitelist.js': 'assets/module-whitelist-b2b974ffee53efecae932353853cf83a.js',
'assets/vendor.css': 'assets/vendor-d41d8cd98f00b204e9800998ecf8427e.css',
'assets/vendor.js': 'assets/vendor-eb3d3e03fa845747687b14602e40e459.js' },
prepend: 'http://cdn.example.com/' }
```

You instead get the prepended URL added to each path:

``` js
{ assets:
{ 'http://cdn.example.com/assets/module-whitelist.css': 'assets/module-whitelist-d41d8cd98f00b204e9800998ecf8427e.css',
'http://cdn.example.com/assets/module-whitelist.js': 'assets/module-whitelist-b2b974ffee53efecae932353853cf83a.js',
'http://cdn.example.com/assets/vendor.css': 'assets/vendor-d41d8cd98f00b204e9800998ecf8427e.css',
'http://cdn.example.com/assets/vendor.js': 'assets/vendor-eb3d3e03fa845747687b14602e40e459.js' },
prepend: 'http://cdn.example.com/' }
```