Use long for expires and lastModified in HeaderAssertions

This commit changes the type of parameters so that HeaderAssertions
can assert expires and lastModified properly.

Issue: SPR-17194

spring-test/src/test/java/org/springframework/test/web/reactive/server/HeaderAssertionTests.java

@@ -17,6 +17,8 @@
 package org.springframework.test.web.reactive.server;
 
 import java.net.URI;
+import java.time.ZoneId;
+import java.time.ZonedDateTime;
 import java.util.concurrent.TimeUnit;
 
 import org.junit.Test;
@@ -217,6 +219,37 @@ public class HeaderAssertionTests {
 		}
 	}
 
+	@Test
+	public void expires() {
+		HttpHeaders headers = new HttpHeaders();
+		ZonedDateTime expires = ZonedDateTime.of(2018, 1, 1, 0, 0, 0, 0, ZoneId.of("UTC"));
+		headers.setExpires(expires);
+		HeaderAssertions assertions = headerAssertions(headers);
+		assertions.expires(expires.toInstant().toEpochMilli());
+		try {
+			assertions.expires(expires.toInstant().toEpochMilli() + 1);
+			fail("Wrong value expected");
+		}
+		catch (AssertionError error) {
+			// Expected
+		}
+	}
+
+	@Test
+	public void lastModified() {
+		HttpHeaders headers = new HttpHeaders();
+		ZonedDateTime lastModified = ZonedDateTime.of(2018, 1, 1, 0, 0, 0, 0, ZoneId.of("UTC"));
+		headers.setLastModified(lastModified.toInstant().toEpochMilli());
+		HeaderAssertions assertions = headerAssertions(headers);
+		assertions.lastModified(lastModified.toInstant().toEpochMilli());
+		try {
+			assertions.lastModified(lastModified.toInstant().toEpochMilli() + 1);
+			fail("Wrong value expected");
+		}
+		catch (AssertionError error) {
+			// Expected
+		}
+	}
 
 	private HeaderAssertions headerAssertions(HttpHeaders responseHeaders) {
 		MockClientHttpRequest request = new MockClientHttpRequest(HttpMethod.GET, URI.create("/"));