Fix inspect/toJSON methods on structures to only omit undefined values, not all falsy ones

[eritbh]

Should be self-explanatory. Also:

• Fixed a typo which would prevent editedTimestamp from showing up in the output of Message#toJSON().
• Fixed Message#editedTimestamp being NaN instead of null on unedited messages

[LJNeon]

I wouldn't be surprised at all if Message#editedTimestamp wasn't a one time thing, maybe make sure the same problem doesn't occur anywhere else?

[eritbh]

Quick GitHub search for editTimestamp doesn't show any other results, I can do a better full check tomorrow