Set Jackson default root value separator to an empty String

JsonGenerator are now reused instead of creating a new one every time. The consequence is the same JsonGenerator is used to serialize multiple events as "root" object.
This commit forces the default root value separator to empty string to avoid extra padding between log events.

Author: brenuart

src/main/java/net/logstash/logback/composite/AbstractCompositeJsonFormatter.java

@@ -30,6 +30,7 @@
 
 import ch.qos.logback.access.spi.IAccessEvent;
 import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.CoreConstants;
 import ch.qos.logback.core.spi.ContextAware;
 import ch.qos.logback.core.spi.ContextAwareBase;
 import ch.qos.logback.core.spi.DeferredProcessingAware;
@@ -39,6 +40,7 @@
 import com.fasterxml.jackson.core.JsonFactory;
 import com.fasterxml.jackson.core.JsonFactory.Feature;
 import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.core.io.SerializedString;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.SerializationFeature;
 
@@ -300,7 +302,14 @@ private JsonGenerator decorateGenerator(JsonGenerator generator) {
                 * Don't let the json generator close the underlying outputStream and let the
                 * encoder managed it.
                 */
-               .disable(JsonGenerator.Feature.AUTO_CLOSE_TARGET);
+               .disable(JsonGenerator.Feature.AUTO_CLOSE_TARGET)
+
+               /*
+                * JsonGenerator are reused to serialize multiple log events.
+                * Change the default roo tvalue separator to an empty string.
+                */
+               .setRootValueSeparator(new SerializedString(CoreConstants.EMPTY_STRING));
+
     }
 
     public JsonFactory getJsonFactory() {