if setting a span to null, call remove()

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/TraceContextHolder.java

@@ -32,7 +32,11 @@ public class TraceContextHolder {
 	}
 
 	public static void setCurrentSpan(Span span) {
-		Assert.notNull(span, "span can not be null.  Use removeCurrentSpan() instead.");
+		// backwards compatibility
+		if (span == null) {
+			currentSpan.remove();
+			return;
+		}
 		if (log.isTraceEnabled()) {
 			log.trace("Setting current span " + span);
 		}

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/TraceContextHolderTests.java

@@ -1,30 +0,0 @@
-/*
- * Copyright 2013-2015 the original author or authors.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.springframework.cloud.sleuth;
-
-import org.junit.Test;
-
-/**
- * @author Spencer Gibb
- */
-public class TraceContextHolderTests {
-
-	@Test(expected = IllegalArgumentException.class)
-	public void setCurrentSpanNotNull() {
-		TraceContextHolder.setCurrentSpan(null);
-	}
-}