Bad behaviour of OSX export from command line

[frankiezafe]

Godot version:
Godot version 3.1.1

OS/device including version:
Linux Mint 19 Cinnamon

Issue description:

When setting the export path extension to .zip, the platform is not taken into account and the behaviour is the same as if --export "pck" is specified.

Same behaviour for all platforms ( --export "Linux/X11", --export "Mac OSX" or --export "Windows Desktop" ), but error appears more often with Mac OSX, as the export generates an archive containing the .app.

Steps to reproduce:

Godot_v3.1.1-stable_x11.64 -v --export "Mac OSX" ../test.zip

[akien-mga]

Duplicate of #23073 (fixed in 3.2).