Cherry-pick commits from researchgate upstream (#15)

* Fixed property visibility.

As seen [here](https://github.com/researchgate/avro-php/blob/ff8841585ad76acbff5b673bafd5b70f35d311cb/lib/avro/data_file.php#L451) this property has to be public

* Fixed schema validation for defaults

Co-authored-by: Siad Ardroumli <siad.ardroumli@gmail.com>

Author: tPl0ch

lib/avro/datum.php

@@ -76,7 +76,7 @@ class AvroIODatumWriter
    * Schema used by this instance to write Avro data.
    * @var AvroSchema
    */
-  private $writers_schema;
+  public $writers_schema;
 
   /**
    * @param AvroSchema $writers_schema

lib/avro/schema.php

@@ -459,9 +459,11 @@ public static function is_valid_datum($expected_schema, $datum)
       case self::REQUEST_SCHEMA:
         if (is_array($datum))
         {
-          foreach ($expected_schema->fields() as $field)
-            if (!array_key_exists($field->name(), $datum) || !self::is_valid_datum($field->type(), $datum[$field->name()]))
-              return false;
+          foreach ($expected_schema->fields() as $field) {
+              $value = isset($datum[$field->name()]) ? $datum[$field->name()] : $field->default_value();
+              if (!self::is_valid_datum($field->type(), $value))
+                  return false;
+          }
           return true;
         }
         return false;