Fix "scale" check on wheel macro (#88)

Author: NielsH12

diffbot_description/urdf/include/common_macros.urdf.xacro

@@ -36,7 +36,7 @@
           <xacro:if value="${wheel_props['scale'] == '' }" >
             <mesh filename="package://${package_name}/meshes/${robot_name}/${wheel_props['mesh']}" />
           </xacro:if>
-          <xacro:if value="${wheel_props['scale'] == '' }" >
+          <xacro:if value="${wheel_props['scale'] != '' }" >
             <mesh filename="package://${package_name}/meshes/${robot_name}/${wheel_props['mesh']}" scale="${wheel_props['scale']}" />
           </xacro:if>
         </geometry>
@@ -341,4 +341,4 @@
   </material>
 <!-- ========================================================================-->
 
-</robot>
+</robot>