Set forgiving encoding fallback when parsing Po file

Before reading the expected encoding from the file itself, we first
try to read it as UTF-8 so we can at least get to read the line that
specifies the encoding.

For some reason, $PerlIO::encoding::fallback is set to FB_CROAK when
we reach this part of the code so the program would instead exit
at the first unknown character.

Unless we were requested a specific encoding, do not even print any
warning about it.

lib/Locale/Po4a/Po.pm

@@ -318,7 +318,9 @@ sub read {
     my $filename = shift
       or croak wrap_mod( "po4a::po", dgettext( "po4a", "Please provide a non-null filename" ) );
 
-    my $charset = shift // 'UTF-8';
+    my $charset = shift;
+    my $did_request_charset = defined($charset);
+    $charset //= 'UTF-8';
     $charset = 'UTF-8' if $charset eq "CHARSET";
     warn "Read $filename with encoding: $charset" if $debug{'encoding'};
 
@@ -340,6 +342,12 @@ sub read {
           unless ( $? == 0 );
     }
 
+    # Allow reading with the wrong encoding.
+    # We will read the expected encoding from the file itself later.
+    use Encode qw(:fallback_all);
+    use PerlIO::encoding;
+    $PerlIO::encoding::fallback = $did_request_charset ? WARN_ON_ERR : FB_DEFAULT;
+
     my $fh;
     if ( $filename eq '-' ) {
         $fh = *STDIN;