don't clobber standard hash attributes in subclasses that have their own hash attributes.

Author: plicease

lib/Module/Build/Base.pm

@@ -763,10 +763,7 @@ sub ACTION_config_data {
   }
 
   sub hash_properties {
-    for (shift->_mb_classes) {
-      return @{$additive_properties{$_}->{'HASH'}}
-        if exists $additive_properties{$_}->{'HASH'};
-    }
+    map { exists $additive_properties{$_}->{HASH} ? @{$additive_properties{$_}->{HASH}} : () } shift->_mb_classes;
   }
 
   sub add_property {

t/add_property_hash.t

@@ -0,0 +1,16 @@
+#!/usr/bin/perl -w
+
+use strict;
+use lib 't/lib';
+use MBTest tests => 1;
+
+blib_load 'Module::Build';
+
+ADDPROP: {
+  package My::Build::Prop;
+  use base 'Module::Build';
+  __PACKAGE__->add_property( 'hash_property' => {});
+}
+
+ok grep { $_ eq 'install_path' } My::Build::Prop->hash_properties, "has install_path even after adding another hash property";
+