Only add .coffee to extensions if it was present before

[tschaub]

The restoreExtensions() function currently results in a '.coffee': undefined member in require.extensions.

On Node 5, if I have a script like the one below (where index.js is empty), I get the output below:

var rewire = require('rewire');
console.log(require.extensions);
rewire('./index.js');
console.log(require.extensions);

{ '.js': [Function], '.json': [Function], '.node': [Function] }
{ '.js': [Function],
  '.json': [Function],
  '.node': [Function],
  '.coffee': undefined }

So, no .coffee before, and '.coffee': undefined after.

The changes in this branch make it so require.extensions is not modified if .coffee is not present in the original.

[coveralls]

Coverage remained the same at 100.0% when pulling 5561b8e on tschaub:unmodified-extensions into f126dcb on jhnns:master.

[tschaub]

@jhnns - let me know if I can provide any more detail or do anything else to help get this merged.

[jhnns]

Thx 👍 Has been shipped with 2.5.2