Support ipV6 Host addresses in HttpHeaders

This commit parses the "Host" HTTP request header as an
`InetSocketAddress`, while supporting IPv6 addresses like `[::1]`.

This host string contains `:` chars even though it has no port
information.

Issue: SPR-15799

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -971,7 +971,12 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 		if (value == null) {
 			return null;
 		}
-		int idx = value.lastIndexOf(':');
+		final int idx;
+		if (value.startsWith("[")) {
+			idx = value.indexOf(':', value.indexOf(']'));
+		} else {
+			idx = value.lastIndexOf(':');
+		}
 		String hostname = null;
 		int port = 0;
 		if (idx != -1 && idx < value.length() - 1) {

spring-web/src/test/java/org/springframework/http/HttpHeadersTests.java

@@ -168,6 +168,14 @@ public class HttpHeadersTests {
 		assertEquals("Invalid Host header", "localhost", headers.getFirst("Host"));
 	}
 
+	@Test
+	public void ipv6Host() {
+		InetSocketAddress host = InetSocketAddress.createUnresolved("[::1]", 0);
+		headers.setHost(host);
+		assertEquals("Invalid Host header", host, headers.getHost());
+		assertEquals("Invalid Host header", "[::1]", headers.getFirst("Host"));
+	}
+
 	@Test(expected = IllegalArgumentException.class)
 	public void illegalETag() {
 		String eTag = "v2.6";