added $unset to UpdateBuilder

spring-data-mongodb/src/main/java/org/springframework/data/document/mongodb/UpdateBuilder.java

@@ -30,6 +30,11 @@ public class UpdateBuilder {
 		return this;
 	}
 
+	public UpdateBuilder unset(String key) {
+		criteria.put("$unset", Collections.singletonMap(key, 1));
+		return this;
+	}
+
 	public UpdateBuilder inc(String key, long inc) {
 		criteria.put("$inc", Collections.singletonMap(key, inc));
 		return this;

spring-data-mongodb/src/test/java/org/springframework/data/document/mongodb/UpdateBuilderTests.java

@@ -43,5 +43,11 @@ public class UpdateBuilderTests {
 				ub.build().toString());
 	}
 
+	@Test
+	public void testUnset() {
+		UpdateBuilder ub = new UpdateBuilder()
+			.unset("directory");
+		Assert.assertEquals("{ \"$unset\" : { \"directory\" : 1}}", ub.build().toString());
+	}
 
 }