Added unit test.

Don't install Bridge if already installed.

jul-to-slf4j/src/main/java/org/slf4j/bridge/osgi/BridgeActivator.java

@@ -34,7 +34,9 @@ import org.slf4j.bridge.SLF4JBridgeHandler;
 public class BridgeActivator implements BundleActivator {
 
   public void start(BundleContext context) throws Exception {
-    SLF4JBridgeHandler.install();
+    if (!SLF4JBridgeHandler.isInstalled()) {
+      SLF4JBridgeHandler.install();
+    }
   }
 
   public void stop(BundleContext context) throws Exception {

jul-to-slf4j/src/test/java/org/slf4j/bridge/osgi/BridgeActivatorTest.java

@@ -0,0 +1,47 @@
+/**
+ * Copyright (c) 2015 QOS.ch
+ * All rights reserved.
+ *
+ * Permission is hereby granted, free  of charge, to any person obtaining
+ * a  copy  of this  software  and  associated  documentation files  (the
+ * "Software"), to  deal in  the Software without  restriction, including
+ * without limitation  the rights to  use, copy, modify,  merge, publish,
+ * distribute,  sublicense, and/or sell  copies of  the Software,  and to
+ * permit persons to whom the Software  is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The  above  copyright  notice  and  this permission  notice  shall  be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE  SOFTWARE IS  PROVIDED  "AS  IS", WITHOUT  WARRANTY  OF ANY  KIND,
+ * EXPRESS OR  IMPLIED, INCLUDING  BUT NOT LIMITED  TO THE  WARRANTIES OF
+ * MERCHANTABILITY,    FITNESS    FOR    A   PARTICULAR    PURPOSE    AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE,  ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package org.slf4j.bridge.osgi;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.slf4j.bridge.SLF4JBridgeHandler;
+
+public class BridgeActivatorTest {
+
+  BridgeActivator classUnderTest = new BridgeActivator();
+
+  @Test
+  public void testActivator() throws Exception {
+    Assert.assertFalse(SLF4JBridgeHandler.isInstalled());
+
+    classUnderTest.start(null);
+
+    Assert.assertTrue(SLF4JBridgeHandler.isInstalled());
+
+    classUnderTest.stop(null);
+
+    Assert.assertFalse(SLF4JBridgeHandler.isInstalled());
+  }
+}