fix(Member): Parse Member#premiumSince to timestamp

[bsian03]

Fixes #1316

[DonovanDMC]

premium_since is defined as both optional & nullable in the docs, optional is handled, nullable is not -- this results in NaN

[bsian03]

What if you don't want a number smh

[JustCat80]

in that case perfect because it is not, luck with your project 👍

[danny-may]

From what im reading in the docs, cant the value be null, undefined or a string? Shouldnt the type guard be a nullish check or a typeof x === 'string' check? Strict null equality would break I believe

[bsian03]

But.. we already have an undefined check

[danny-may]

Ah, on the lines above, I missed that. Ignore me :)

stale

[DonovanDMC]

typings (nullable & optional on both Member & OldMember)

eris/index.d.ts

Line 654 in b2a90ea

premiumSince: number;

eris/index.d.ts

Line 3046 in b2a90ea

premiumSince: number;