rebase: bump golang.org/x/net from 0.2.0 to 0.4.0

Bumps [golang.org/x/net](https://github.com/golang/net) from 0.2.0 to 0.4.0.
- [Release notes](https://github.com/golang/net/releases)
- [Commits](https://github.com/golang/net/compare/v0.2.0...v0.4.0)

---
updated-dependencies:
- dependency-name: golang.org/x/net
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

vendor/golang.org/x/net/http2/server.go

@@ -588,6 +588,7 @@ type serverConn struct {
 	maxFrameSize                int32
 	peerMaxHeaderListSize       uint32            // zero means unknown (default)
 	canonHeader                 map[string]string // http2-lower-case -> Go-Canonical-Case
+	canonHeaderKeysSize         int               // canonHeader keys size in bytes
 	writingFrame                bool              // started writing a frame (on serve goroutine or separate)
 	writingFrameAsync           bool              // started a frame on its own goroutine but haven't heard back on wroteFrameCh
 	needsFrameFlush             bool              // last frame write wasn't a flush
@@ -766,6 +767,13 @@ func (sc *serverConn) condlogf(err error, format string, args ...interface{}) {
 	}
 }
 
+// maxCachedCanonicalHeadersKeysSize is an arbitrarily-chosen limit on the size
+// of the entries in the canonHeader cache.
+// This should be larger than the size of unique, uncommon header keys likely to
+// be sent by the peer, while not so high as to permit unreasonable memory usage
+// if the peer sends an unbounded number of unique header keys.
+const maxCachedCanonicalHeadersKeysSize = 2048
+
 func (sc *serverConn) canonicalHeader(v string) string {
 	sc.serveG.check()
 	buildCommonHeaderMapsOnce()
@@ -781,14 +789,10 @@ func (sc *serverConn) canonicalHeader(v string) string {
 		sc.canonHeader = make(map[string]string)
 	}
 	cv = http.CanonicalHeaderKey(v)
-	// maxCachedCanonicalHeaders is an arbitrarily-chosen limit on the number of
-	// entries in the canonHeader cache. This should be larger than the number
-	// of unique, uncommon header keys likely to be sent by the peer, while not
-	// so high as to permit unreasonable memory usage if the peer sends an unbounded
-	// number of unique header keys.
-	const maxCachedCanonicalHeaders = 32
-	if len(sc.canonHeader) < maxCachedCanonicalHeaders {
+	size := 100 + len(v)*2 // 100 bytes of map overhead + key + value
+	if sc.canonHeaderKeysSize+size <= maxCachedCanonicalHeadersKeysSize {
 		sc.canonHeader[v] = cv
+		sc.canonHeaderKeysSize += size
 	}
 	return cv
 }