Fix MockHttpServletRequest.setCookies to produce single cookie header

Prior to this commit, MockHttpServletRequest.setCookies() produced one
Cookie header per supplied cookie, resulting in multiple Cookie headers
which violates the specification.

This commit fixes this by ensuring that all cookie name-value pairs are
stored under a single Cookie header, separated by a semicolon.

Closes gh-23074

spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java

@@ -292,7 +292,9 @@ public class MockHttpServletRequestTests {
 		assertEquals("bar", cookies[0].getValue());
 		assertEquals("baz", cookies[1].getName());
 		assertEquals("qux", cookies[1].getValue());
-		assertEquals(Arrays.asList("foo=bar", "baz=qux"), cookieHeaders);
+
+		assertEquals(1, cookieHeaders.size());
+		assertEquals("foo=bar; baz=qux", cookieHeaders.get(0));
 	}
 
 	@Test