Preserving bpack.hasExports on reset()

[JedWatson]

I went digging to figure out what was causing #887 and found that the hasExports setting for browser-pack is externally managed by browserify under various conditions, including the b.require() method.

Using watchify, you call .bundle() again on update which internally calls .reset() to recreate the pipeline, which creates a new this._bpack instance.

Because hasExports is not set in this._options, the hasExternal setting never makes it back into the new _bpack instance, meaning subsequent bundles behave differently.

I've fixed it by remembering the setting immediately before the pipeline is recreated, and immediately afterwards reapplying the previous value.

I considered adding it to the opts provided to this._createPipeline but didn't want to introduce any side effects, the implementation I went with is more consistent with the way this._bpack.hasExports is set in all the other cases, so it will now have exactly the same behaviour initially and after any pipeline reset.

[ghost]

Thanks for the patch! I added a test case and shipped this with 5.11.1.

[JedWatson]

That was fast! Thanks :)