Give StringBuilder an initial capacity of 256 (#150)

closes #149

Author: echatman

logback-ecs-encoder/src/main/java/co/elastic/logging/logback/EcsEncoder.java

@@ -95,7 +95,7 @@ public void close() throws IOException {
 
     @Override
     public byte[] encode(ILoggingEvent event) {
-        StringBuilder builder = new StringBuilder();
+        StringBuilder builder = new StringBuilder(256);
         EcsJsonSerializer.serializeObjectStart(builder, event.getTimeStamp());
         EcsJsonSerializer.serializeLogLevel(builder, event.getLevel().toString());
         EcsJsonSerializer.serializeFormattedMessage(builder, event.getFormattedMessage());