Add ClientRequest.attribute(String)

This commit adds the attribute(String) convenience method to the
ClientRequest class. It also adapts the Basic Authentication filter to
use it.

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/ExchangeFilterFunctionsTests.java

@@ -117,4 +117,20 @@ public class ExchangeFilterFunctionsTests {
 		assertEquals(response, result);
 	}
 
+	@Test
+	public void basicAuthenticationAbsentAttributes() throws Exception {
+		ClientRequest request = ClientRequest.method(GET, URI.create("http://example.com")).build();
+		ClientResponse response = mock(ClientResponse.class);
+
+		ExchangeFunction exchange = r -> {
+			assertFalse(r.headers().containsKey(HttpHeaders.AUTHORIZATION));
+			return Mono.just(response);
+		};
+
+		ExchangeFilterFunction auth = ExchangeFilterFunctions.basicAuthentication();
+		assertFalse(request.headers().containsKey(HttpHeaders.AUTHORIZATION));
+		ClientResponse result = auth.filter(request, exchange).block();
+		assertEquals(response, result);
+	}
+
 }