DATAREDIS-1119 - Polishing.

Add getAndMap(…) method to simplify post-retrieval mapping of non-null values. Require key on XInfoCommand command construction.

Tweak javadoc. Reformat code.

Original pull request: #519.

src/test/java/org/springframework/data/redis/connection/AbstractConnectionIntegrationTests.java

@@ -3339,7 +3339,6 @@ public abstract class AbstractConnectionIntegrationTests {
 		XInfoGroups info = (XInfoGroups) results.get(2);
 
 		assertThat(info.size()).isZero();
-
 	}
 
 	@Test // DATAREDIS-1119