Align default values for conductor_bsdf with specification (#1379)

The default values on the nodedef for conductor_bsdf was by mistake setup for Copper while the specification has values set for Gold. This change list fixes this by changing the nodedef values to align with the specification.

libraries/pbrlib/pbrlib_defs.mtlx

@@ -76,8 +76,8 @@
   -->
   <nodedef name="ND_conductor_bsdf" node="conductor_bsdf" bsdf="R" nodegroup="pbr" doc="A reflection BSDF node based on a microfacet model and a Fresnel curve for conductors/metals.">
     <input name="weight" type="float" value="1.0" uimin="0.0" uimax="1.0" />
-    <input name="ior" type="color3" value="0.271, 0.677, 1.316" colorspace="none" />
-    <input name="extinction" type="color3" value="3.609, 2.625, 2.292" colorspace="none" />
+    <input name="ior" type="color3" value="0.183, 0.421, 1.373" colorspace="none" />
+    <input name="extinction" type="color3" value="3.424, 2.346, 1.770" colorspace="none" />
     <input name="roughness" type="vector2" value="0.05, 0.05" />
     <input name="normal" type="vector3" defaultgeomprop="Nworld" />
     <input name="tangent" type="vector3" defaultgeomprop="Tworld" />