add isConnected() methods to logger and sender; enhance tests with this functionality

Author: Mike Kobyakov

src/main/java/org/fluentd/logger/FluentLogger.java

@@ -128,4 +128,8 @@ public void finalize() {
             sender.close();
         }
     }
+
+    public boolean isConnected() {
+        return sender != null && sender.isConnected();
+    }
 }

src/main/java/org/fluentd/logger/sender/NullSender.java

@@ -50,4 +50,9 @@ public String getName() {
     public String toString() {
         return this.getClass().getName();
     }
+
+    @Override
+    public boolean isConnected() {
+        return true;
+    }
 }

src/main/java/org/fluentd/logger/sender/RawSocketSender.java

@@ -211,4 +211,9 @@ public String getName() {
     public String toString() {
         return getName();
     }
+
+    @Override
+    public boolean isConnected() {
+        return socket != null && !socket.isClosed() && socket.isConnected() && !socket.isOutputShutdown();
+    }
 }

src/main/java/org/fluentd/logger/sender/Sender.java

@@ -29,4 +29,6 @@ public interface Sender {
     void close();
 
     String getName();
+
+    boolean isConnected();
 }