Fix handling of heartbeat gateway event (#3105) #952

Job	Run time
Check minimal versions	1m 3s
Test (macOS)	3m 35s
MSRV	1m 5s
Test (nightly)	2m 58s
Test (stable)	2m 47s
Test (beta)	2m 55s
Build docs	1m 1s
Test (Windows)	5m 21s
Examples	4m 13s
Test (native-tls)	2m 37s
Test (no default features)	53s
Test (no cache)	2m 31s
Test (no gateway)	1m 51s
Test (unstable Discord API features)	2m 38s
Test (simd-json)	2m 46s
Test (chrono, chrono)	49s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 19s
Test (builder without model, builder, true)	36s
Test (unstable Discord API (no default features), unstable_discord_api, true)	16s
	41m 14s

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