Flush silent Buffers by default

With the change to allow ading fields to the Buffer with a Logger
interface in the previous commit, we should also flush those Buffers
with fields set that way by default.

lib/rackstash/buffer.rb

@@ -79,7 +79,7 @@ module Rackstash
     #   were just added fields or tags without any logged messages. If this is
     #   `false` and there were no messages logged with {#add_message}, the
     #   buffer will not be flushed to the sink but will be silently dropped.
-    def initialize(sink, buffering: true, allow_silent: false)
+    def initialize(sink, buffering: true, allow_silent: true)
       @sink = sink
       @buffering = !!buffering
       @allow_silent = !!allow_silent

spec/rackstash/buffer_spec.rb

@@ -14,8 +14,8 @@ describe Rackstash::Buffer do
   let(:buffer) { described_class.new(sink, **buffer_options) }
 
   describe '#allow_silent?' do
-    it 'defaults to false' do
-      expect(buffer.allow_silent?).to be false
+    it 'defaults to true' do
+      expect(buffer.allow_silent?).to be true
     end
   end
 
@@ -101,6 +101,28 @@ describe Rackstash::Buffer do
       buffer.add_fields(key: 'value')
     end
 
+    context 'when allow_silent?' do
+      before do
+        buffer_options[:allow_silent] = true
+      end
+
+      it 'sets pending? to true' do
+        buffer.add_fields(key: 'value')
+        expect(buffer.pending?).to be true
+      end
+    end
+
+    context 'when not allow_silent?' do
+      before do
+        buffer_options[:allow_silent] = false
+      end
+
+      it 'does not set pending? to true' do
+        buffer.add_fields(key: 'value')
+        expect(buffer.pending?).to be false
+      end
+    end
+
     context 'when buffering?' do
       before do
         buffer_options[:buffering] = true