Ruby opaque pointer handling regression fix

This bug was introduced in swig-3.0.8 in #146252 adding shared_ptr
support. An ObjectPreviouslyDeleted error was incorrectly thrown
when the pointer was used as a parameter after being set to zero
via a call to 'DATA_PTR(self) = 0'.

It isn't clear to me which approach is better in this corner case,
so I've gone for backwards compatibility and restored the old behaviour.

Closes #602

Examples/test-suite/ruby/Makefile.in

@@ -30,7 +30,8 @@ CPP_TEST_CASES = \
 #	stl_new
 
 C_TEST_CASES += \
-	li_cstring
+	li_cstring \
+	ruby_manual_proxy \
 
 include $(srcdir)/../common.mk
 

Examples/test-suite/ruby/ruby_manual_proxy_runme.rb

@@ -0,0 +1,49 @@
+#!/usr/bin/env ruby
+#
+# The Subversion bindings use this manually written proxy class approach
+# to the Ruby bindings. Note that in C the struct svn_fs_t is an
+# opaque pointer and the Ruby FileSystem proxy class is hand written around it.
+# This testcase tests this and the C close function and subsequent error
+# handling.
+
+require 'swig_assert'
+require 'ruby_manual_proxy'
+
+module Svn
+  module Fs
+    module_function
+    def create(path)
+      f = Ruby_manual_proxy::svn_fs_create(path)
+      return f
+    end
+
+    FileSystem = SWIG::TYPE_p_svn_fs_t
+    class FileSystem
+      class << self
+        def create(*args)
+          Fs.create(*args)
+        end
+      end
+      def path
+        Ruby_manual_proxy::svn_fs_path(self)
+      end
+    end
+  end
+end
+
+f = Svn::Fs::FileSystem.create("/tmp/myfile")
+path = f.path
+f.close
+begin
+  # regression in swig-3.0.8 meant ObjectPreviouslyDeleted error was thrown instead
+  path = f.path
+  raise RuntimeError.new("IOError (1) not thrown")
+rescue IOError
+end
+
+file = nil
+begin
+  path = Ruby_manual_proxy::svn_fs_path(file)
+  raise RuntimeError.new("IOError (2) not thrown")
+rescue IOError
+end