AVRO-1830 [Perl] Support containers without codec

As [the specification on Object Container Files][spec] states
(emphasis added):

> All metadata properties that start with "avro." are reserved.
> The following file metadata properties are currently used:
>
>   * **avro.schema** contains the schema of objects stored in
>     the file, as JSON data (required).
>   * **avro.codec** the name of the compression codec used to
>     compress blocks, as a string. Implementations are required
>     to support the following codecs: "null" and "deflate". _If
>     codec is absent, it is assumed to be "null"_. The codecs are
>     described with more detail below.

This change makes it so that the Perl implementation does not die
when opening a container that does not contain an explicit codec
in its metadata.

This change is inspired by one originally submitted in 2016 by
SK Liew.

[spec]: https://avro.apache.org/docs/1.11.1/specification/#object-container-files

lang/perl/Changes

@@ -4,6 +4,9 @@ Revision history for Perl extension Avro
           multiple JSON backends
         - Drop dependency on IO::String, since we don't need
           it now we depend on Perl 5.10.1 or greater
+        - Support object containers without an explicit
+          codec. It will be assumed to be 'null' as mandated
+          by the spec.
 
 1.00  Fri Jan 17 15:00:00 2014
         - Relicense under apache license 2.0

lang/perl/lib/Avro/DataFileReader.pm

@@ -55,7 +55,7 @@ sub new {
 
 sub codec {
     my $datafile = shift;
-    return $datafile->metadata->{'avro.codec'};
+    return $datafile->metadata->{'avro.codec'} || 'null';
 }
 
 sub writer_schema {
@@ -99,7 +99,7 @@ sub read_file_header {
     $datafile->{sync_marker} = $data->{sync}
         or croak "sync marker appears invalid";
 
-    my $codec = $data->{meta}{'avro.codec'} || "";
+    my $codec = $data->{meta}{'avro.codec'} || 'null';
 
     throw Avro::DataFile::Error::UnsupportedCodec($codec)
         unless Avro::DataFile->is_codec_valid($codec);

lang/perl/t/04_datafile.t

@@ -221,4 +221,21 @@ is_deeply $all[0], $data, "Our data is intact!";
     is scalar @next, 0, "no more objects back";
 }
 
+## Test with a datafile that has no codec
+{
+    my $container = join '',
+        "Obj\x{01}",
+        "\x{02}\x{16}avro.schema\x{10}\x{22}string\x{22}\x{00}",
+        "\x{de}\x{ad}\x{be}\x{ef}" x 4,
+        "\x{02}\x{08}\x{06}foo",
+        "\x{de}\x{ad}\x{be}\x{ef}" x 4;
+
+    open my $fh, '<', \$container or die "Could not open memory handle: $!";
+
+    my $reader = Avro::DataFileReader->new( fh => $fh );
+
+    my ($data) = $reader->next(1);
+    is $data, 'foo', 'Can read data from container without a codec';
+}
+
 done_testing;