Fix detection of Option + Space key on macOS

[WesleyKapow]

The Option + Space key combination on macOS results in a non-breaking
space (U+00A0). This resulted in the modifiers not correctly detecting
the release of the space key.

To reproduce on a mac:

• Log event.modifiers.space: http://sketch.paperjs.org/#V/0.12.4/S/JcwxDoJAEEbhq0ymksTsATAegcpSLNZh0A3L/IRZaAh3B2P5vuJtbHFUrvkxaJEvX1nQ/boAOcAaLK4NVqU79YtJSbCLrmqloq01IoE5soaMz9/DiC71SWcPPkXR6tbafn7fs8ZhQrLiXD9f+wE=
• Click into canvas to give it focus (otherwise space may enter a space somewhere else)
• Continuously move mouse around to generate console outputs.
• Hold down space key.
• While still holding space, hold down option key.
• While still holding option key, release space key.

Expected:

• Console log switches back to false.

Actual:

• Console log continues to log true as the modifier is "stuck".