[types] permit undefined createContext initial value

[developit]

No description provided.

[coveralls]

Coverage remained the same at 99.625% when pulling b6053a1 on types-createcontext-default-value into d4a3013 on master.

[developit]

Or maybe this?

Suggested change

	export function createContext<T = undefined>(defaultValue: T): Context<T>;
	export function createContext<T>(defaultValue: T): Context<T>;
	export function createContext<T extends undefined>(defaultValue?: T): Context<T>;