bump_scale should return a float

utils/exporters/blender/addons/io_three/exporter/api/material.py

@@ -70,7 +70,10 @@ def bump_scale(material):
     :rtype: float
 
     """
-    return normal_scale(material)
+    logger.debug("material.bump_scale(%s)", material)
+    for texture in _valid_textures(material):
+        if texture.use_map_normal:
+            return texture.normal_factor
 
 
 @_material