Add flexibility on mtl properties case

```
Some tools generate mtl files with wrong case for properties like "map_Kd". The
property name generated is "map_kd".
Example : the great tool assimp does that : http://assimp.sourceforge.net/

Checking the porperty name in lower case fixes that problem (even if it's not
really abug).

A patch is attached.
```

Original issue reported on code.google.com by `laurent....@fabzat.com` on 10 Apr 2014 at 1:22

Attachments:
- [fix_obj_case.patch](https://storage.googleapis.com/google-code-attachments/jsc3d/issue-71/comment-0/fix_obj_case.patch)