fix(WebsocketShard): pass ignoreACK into sendHeartbeat when Discord asks for one

[didinele]

Please describe the changes this PR makes and why it should be merged:
Currently when Discord sends a "HEARTBEAT" payload (opcode 1) discord.js simply calls sendHeartbeat with the appropriate label, without actually telling it to not check if the last heartbeat ever got the ACK.

What can end up happening in its current state is:

1. d.js sends a regular beat, on the interval
2. Discord asks for another beat using OP 1
3. d.js calls sendHeartbeat yet again which will check if the beat sent on step 1 recieved the ACK; if it has, nothing horrible happens, otherwise d.js will respawn the shard under the impression that we're dealing with a zombie connection.

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[vladfrangu]

This is a minor thing, and it shouldn't really matter, but it makes sense to skip the checks considering Discord asked us for one, LGTM