Minor fix to avoid extra trailing zeroes during serialization (#2521)

Author: rayokota

src/Confluent.SchemaRegistry.Serdes.Avro/GenericSerializerImpl.cs

@@ -182,7 +182,7 @@ public async Task<byte[]> Serialize(string topic, Headers headers, GenericRecord
                         .Write(data, new BinaryEncoder(stream));
 
                     var buffer = await ExecuteRules(isKey, subject, topic, headers, RulePhase.Encoding, RuleMode.Write,
-                            null, latestSchema, stream.GetBuffer(), null)
+                            null, latestSchema, stream.ToArray(), null)
                         .ContinueWith(t => (byte[])t.Result)
                         .ConfigureAwait(continueOnCapturedContext: false);
 

src/Confluent.SchemaRegistry.Serdes.Avro/SpecificSerializerImpl.cs

@@ -223,7 +223,7 @@ public async Task<byte[]> Serialize(string topic, Headers headers, T data, bool
                     currentSchemaData.AvroWriter.Write(data, new BinaryEncoder(stream));
 
                     var buffer = await ExecuteRules(isKey, subject, topic, headers, RulePhase.Encoding, RuleMode.Write,
-                            null, latestSchema, stream.GetBuffer(), null)
+                            null, latestSchema, stream.ToArray(), null)
                         .ContinueWith(t => (byte[])t.Result)
                         .ConfigureAwait(continueOnCapturedContext: false);