fix bug where headers were not allowed to define explode when decoding #694

Job	Run time
linux (swift:5.1-xenial)	5m 11s
linux (swift:5.1-bionic)	4m 56s
linux (swift:5.2-xenial)	6m 16s
linux (swift:5.2-bionic)	8m 10s
linux (swift:5.2-focal)	7m 35s
linux (swift:5.2-centos8)	5m 13s
linux (swift:5.2-amazonlinux2)	6m 52s
linux (swift:5.3-xenial)	3m 46s
linux (swift:5.3-bionic)	3m 35s
linux (swift:5.3-focal)	4m 19s
linux (swift:5.3-centos8)	3m 39s
linux (swift:5.3-amazonlinux2)	4m 23s
linux-5_5-plus (swiftlang/swift:nightly-5.5-xenial)	4m 1s
linux-5_5-plus (swiftlang/swift:nightly-5.5-bionic)	3m 23s
linux-5_5-plus (swiftlang/swift:nightly-5.5-focal)	4m 22s
linux-5_5-plus (swiftlang/swift:nightly-5.5-centos8)	4m 20s
linux-5_5-plus (swiftlang/swift:nightly-5.5-amazonlinux2)	4m 22s
linux-5_5-plus (swiftlang/swift:nightly-xenial)	4m 2s
linux-5_5-plus (swiftlang/swift:nightly-bionic)	4m 30s
linux-5_5-plus (swiftlang/swift:nightly-focal)	3m 30s
linux-5_5-plus (swiftlang/swift:nightly-centos8)	4m 10s
linux-5_5-plus (swiftlang/swift:nightly-amazonlinux2)	4m 40s
linux-5_4 (swift:5.4-xenial)	3m 58s
linux-5_4 (swift:5.4-bionic)	3m 16s
linux-5_4 (swift:5.4-focal)	3m 14s
linux-5_4 (swift:5.4-centos8)	3m 21s
linux-5_4 (swift:5.4-amazonlinux2)	3m 34s
osx	4m 50s
	2h 7m 28s

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