Fix method name escaping in ObjectSpace.dump

It's possible to define methods with any name, even if the parser
doesn't support it and it can only be used with ex. send.

This fixes an issue where invalid JSON was output from ObjectSpace.dump
when a method name needed escaping.

ext/objspace/objspace_dump.c

@@ -313,7 +313,8 @@ dump_object(VALUE obj, struct dump_config *dc)
 	dump_append(dc, ", \"file\":\"%s\", \"line\":%lu", ainfo->path, ainfo->line);
 	if (RTEST(ainfo->mid)) {
 	    VALUE m = rb_sym2str(ainfo->mid);
-	    dump_append(dc, ", \"method\":\"%s\"", RSTRING_PTR(m));
+	    dump_append(dc, ", \"method\":");
+	    dump_append_string_value(dc, m);
 	}
 	dump_append(dc, ", \"generation\":%"PRIuSIZE, ainfo->generation);
     }

test/objspace/test_objspace.rb

@@ -362,6 +362,24 @@ class TestObjSpace < Test::Unit::TestCase
     end
   end
 
+  def test_dump_escapes_method_name
+    method_name = "foo\"bar"
+    klass = Class.new do
+      define_method(method_name) { "TEST STRING" }
+    end
+    ObjectSpace.trace_object_allocations_start
+
+    obj = klass.new.send(method_name)
+
+    dump = ObjectSpace.dump(obj)
+    assert_includes dump, '"method":"foo\"bar"'
+
+    parsed = JSON.parse(dump)
+    assert_equal "foo\"bar", parsed["method"]
+  ensure
+    ObjectSpace.trace_object_allocations_stop
+  end
+
   def test_dump_reference_addresses_match_dump_all_addresses
     assert_in_out_err(%w[-robjspace], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
       begin;