Fix breakage caused by CacheHttp -> Http #1095

Job	Run time
Build docs	1m 28s
Rustfmt	11s
Clippy	42s
Test (MSRV)	1m 13s
Test (stable)	1m 46s
Test (beta)	1m 57s
Test (nightly)	1m 41s
Test (macOS)	1m 31s
Test (Windows)	5m 0s
Test (no features)	1m 1s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 20s
	17m 50s

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