How to reproduce:
1. Open the attached project
2. Open and play test scene
Actual result: notice NullReferenceException in console
Expected result: www.bytes returns bytes and AssetBundle is loaded

Issue doesn't exist on Windows. Also it is not related to AssetBundles, I have tried loading text file, www.bytes still returns null.

Reproducible: 2017.1.0b1
Not reproducible: 2017.1.0a6

By Design:
This is because we made WWW to be a wrapper on top of UnityWebRequest and we are sticter in terms of URL validation.
This line is correct in Windows, but not OSX:
wwwPath = "file:///" + Application.streamingAssetsPath + "/iOS/" +assetBundleName;
because it result in Uri with 4 slashes after file:.

The correct way for this is:
new Uri(Application.streamingAssetsPath + "/iOS/" +assetBundleName, UriKind.Absolute).AbsoluteUri

This would result in proper URI under both Windows and OSX.