gzhttp: Use crc32.Update instead of crc32.New

Skips another allocation:

name        old time/op    new time/op    delta
2kJitter-8    29.4µs ± 4%    29.0µs ± 3%  -1.23%  (p=0.017 n=19+24)

name        old speed      new speed      delta
2kJitter-8  69.8MB/s ± 4%  70.6MB/s ± 3%  +1.23%  (p=0.016 n=19+24)

name        old alloc/op   new alloc/op   delta
2kJitter-8    3.41kB ± 4%    3.34kB ± 4%  -2.12%  (p=0.001 n=20+25)

name        old allocs/op  new allocs/op  delta
2kJitter-8      16.0 ± 0%      15.0 ± 0%  -6.25%  (p=0.000 n=20+25)

gzhttp/compress.go

@@ -234,17 +234,16 @@ func (w *GzipResponseWriter) startGzip(remain []byte) error {
 					var tmp [sha256.Size]byte
 					jitRNG = binary.LittleEndian.Uint32(h.Sum(tmp[:0]))
 				} else {
-					h := crc32.New(castagnoliTable)
-					h.Write(w.buf)
+					h := crc32.Update(0, castagnoliTable, w.buf)
 					// Use only up to "w.jitterBuffer", otherwise the output depends on write sizes.
 					if len(remain) > 0 && len(w.buf) < w.jitterBuffer {
 						remain := remain
 						if len(remain)+len(w.buf) > w.jitterBuffer {
 							remain = remain[:w.jitterBuffer-len(w.buf)]
 						}
-						h.Write(remain)
+						h = crc32.Update(h, castagnoliTable, remain)
 					}
-					jitRNG = bits.RotateLeft32(h.Sum32(), 19) ^ 0xab0755de
+					jitRNG = bits.RotateLeft32(h, 19) ^ 0xab0755de
 				}
 			} else {
 				// Get from rand.Reader