Material Specular attribute is ignored

### Steps to reproduce
- Export a model with a material with any **Specular Intensity** value from Blender
- Import the model in _Godot 3 beta 1_

### Expected behavior
The material will have **metallic_specular** value to be equal to Specular Intensity value from Blender

### Actual behavior
The material **metallic_specular** value is **0.5**


Start by reproducing the Blender-to-Godot export and import flow described in the issue, then trace where Blender's Specular Intensity is read and assigned to Godot's metallic_specular value. Done means exported materials preserve the Blender Specular Intensity instead of always importing as 0.5.

Written by the indexing model from the issue text.