Reinstate RST_STREAM at EOS

src/core/ext/transport/chttp2/transport/chttp2_transport.c

src/core/ext/transport/chttp2/transport/writing.c

src/core/lib/iomgr/combiner.c

src/core/lib/support/log.c

src/core/lib/support/string.c

src/core/lib/support/string.h

test/core/support/string_test.c