Write the raw buffer to the event on flush and use #to_h instead of #to_event to create the event

With that, we can lazy-transform the Buffer to the event hash. B using
the common `.to_h` protocol, we can also support various other objects
here instead of just Buffers (including actual raw Hashes).

lib/rackstash/buffer.rb

@@ -222,18 +222,15 @@ module Rackstash
       @fields ||= Rackstash::Fields::Hash.new(forbidden_keys: FORBIDDEN_FIELDS)
     end
 
-    # Flush the current buffer to the {#flows} if it is pending.
-    #
-    # After the flush, the existing buffer should not be used anymore. You
-    # should either call {#clear} to remove all volatile data or create a new
-    # buffer instance instead.
+    # Flush the current buffer to the {#flows} if it is pending. All data in the
+    # buffer will be preserved. You might call {#clear} to start anew.
     #
     # @return [self,nil] returns `self` if the buffer was flushed, `nil`
     #   otherwise
     def flush
       return unless pending?
 
-      @flows.write(self.to_event)
+      @flows.write(self)
       self
     end
 
@@ -347,7 +344,7 @@ module Rackstash
     # All hashes (including nested hashes) use `String` keys.
     #
     # @return [Hash] the event expected by the event {Filter}s.
-    def to_event
+    def to_h
       event = fields.to_h
       event[FIELD_TAGS] = tags.to_a
       event[FIELD_MESSAGE] = messages

lib/rackstash/flows.rb

@@ -179,6 +179,7 @@ module Rackstash
       flows = to_a
       flows_size = flows.size
 
+      event = event.to_h
       flows.each_with_index do |flow, index|
         # If we have more than one flow, we provide a fresh copy of the event
         # to each flow. The flow's filters and encoder can then mutate the event

spec/rackstash/buffer_spec.rb

@@ -360,12 +360,11 @@ RSpec.describe Rackstash::Buffer do
         buffer.add_message(message)
 
         # We might call Buffer#flush during the following tests
-        allow(buffer).to receive(:to_event).and_return(event)
-        allow(flows).to receive(:write).with(event).once
+        allow(flows).to receive(:write).with(buffer).once
       end
 
       it 'flushes the buffer to the flows' do
-        expect(flows).to receive(:write).with(event).once
+        expect(flows).to receive(:write).with(buffer).once
         buffer.flush
       end
 
@@ -563,7 +562,7 @@ RSpec.describe Rackstash::Buffer do
     end
   end
 
-  describe '#to_event' do
+  describe '#to_h' do
     it 'creates an event hash' do
       message = double(message: 'Hello World', time: Time.now)
       allow(message)
@@ -571,7 +570,7 @@ RSpec.describe Rackstash::Buffer do
       buffer.fields[:foo] = 'bar'
       buffer.tags << 'some_tag'
 
-      expect(buffer.to_event).to match(
+      expect(buffer.to_h).to match(
         'foo' => 'bar',
         'message' => [message],
         'tags' => ['some_tag'],