Missing toString override in GlobalProperties$Keys causes incorrectly named Keys due to string concatenation with existing Keys

[DJtheRedstoner]

The lack of a toString override in GlobalProperties$Keys makes the following code (and other similar instances) create a key similar to org.spongepowered.asm.launch.GlobalProperties$Keys@239b0f9d.default. https://github.com/SpongePowered/Mixin/blob/155314e6e91465dad727e621a569906a410cd6f4/src/main/java/org/spongepowered/asm/mixin/MixinEnvironment.java#L1096

[Mumfrey]

Thanks for opening this, marked as bug.