Changed so uppercase variables passthrough as constants

[vallentin]

Resolves #432

Fixes so that variables including an uppercase letter, doesn't get prefixed self., i.e. {{ FOO }} results in FOO instead of self.FOO.

[vallentin]

This indirectly also fixes so that {{ None }} results in None instead of self.None (that's why I only check for a single uppercase char).

However, {{ Some("foo") }} is also interpreted as a variable it seems. It at least incorrectly generates (self.Some)("foo"). I'll fix that in a future PR.