Open Radar

 
Summary:
When loading a .mtl file with certain properties (e.g. specularityExponent) a property is dropped when the properties are enumerated using Swift subscripting.

Steps to Reproduce:
1) Run the attached project
2) Notice in the debug log that when the properties are enumerate using Swift the property 'specularExponent' is nil, it is not nil when Objective C is used or when the property is queried by name.

Expected Results:
It should be possible to enumerate the properties successfully and accurately via both Swift and Objective C.

Actual Results:
Objective C finds the following properties:
2016-05-25 21:48:35.821 ModelIOSpecularExponent-iOS[21057:1590476] [15]: ambientOcclusionScale
2016-05-25 21:48:35.821 ModelIOSpecularExponent-iOS[21057:1590476] [16]: specularExponent
2016-05-25 21:48:35.821 ModelIOSpecularExponent-iOS[21057:1590476] [17]: opacity
Swift finds the following:
2016-05-25 21:48:35.823 ModelIOSpecularExponent-iOS[21057:1590476] [15]: Optional("ambientOcclusionScale")
2016-05-25 21:48:35.823 ModelIOSpecularExponent-iOS[21057:1590476] [16]: nil
2016-05-25 21:48:35.824 ModelIOSpecularExponent-iOS[21057:1590476] [17]: Optional("opacity")

Regression:
Occurs in all versions of ModelIO of OS X and iOS. Did not find a working version. But this is a Swift vs. Objective C regression.

Notes:
During debugging it was determined that the culpable routine is `-[MDLMaterial objectAtIndexedSubscript:]`.
MDLMaterial maintains two arrays _builtinProperties and _userProperties.
In the example case specularExponent is the first entry in the _userProperties array. 
The method logic incorrectly looks it up in the _builtinProperties.