use AppendHeader for http2 #627

Job	Run time
gcc-compile-with-cmake	5m 18s
gcc-compile-with-bazel	24m 51s
gcc-compile-with-cmake-all-options	7m 18s
gcc-compile-with-make	4m 18s
gcc-compile-with-make-all-options	6m 28s
clang-compile-with-bazel	18m 56s
clang-compile-with-bazel-all-options	23m 4s
clang-compile-with-cmake	4m 45s
clang-compile-with-cmake-all-options	6m 34s
clang-compile-with-make	3m 34s
clang-compile-with-make-all-options	6m 56s
clang-unittest	15m 45s
gcc-compile-with-bazel-all-options	35m 46s
	2h 43m 33s

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