Always apply GLOBAL_specular 1.0 when NORMAL texture given

[tngreene]

Since the NORMAL texture is supplying its own specularity, we want to make sure the OBJ default is to not mess with it and always return to 1.0 for the 1% of people who want to change the specular level of an object with a normal texture.

If has TEXTURE_NORMAL and GLOBAL_specular != 1.0:
    write GLOBAL_specular 1.0

TEXTURE_DRAPED_NORMAL has "non-stupid" defaults and doesn't need this protection.

XPlane2Blender 2.49 shows this behavior with

           if tex_exists(core+"_NML.png"): self.file.write("TEXTURE_NORMAL %s%s_NML.png\nGLOBAL_specular 1.0\n" % (self.prefix,core))

We get this a little correct by inserting this after NORMAL_METALNESS is active

        if xplane_version >= 1100 and self.xplaneFile.referenceMaterials[0]:
            mat = self.xplaneFile.referenceMaterials[0]
            if mat.getEffectiveNormalMetalness():
                self.attributes['GLOBAL_specular'].setValue(1.0)
                self.xplaneFile.commands.written['ATTR_shiny_rat'] = 1.0 # Here we are fooling ourselves
                write_user_specular_values = False #It will be skipped from now on

but it should be for more than NORMAL_METALNESS

[tngreene]

We'll need a unit test for this behavior, but this also affects #443 for what we should be testing for. Therefore I'm giving it the 249 bug label and also scheduled for v3.5.1-beta.2