[Bug #19087] Fix an assertion of `String#to_c`

spec/ruby/core/string/to_c_spec.rb

@@ -24,8 +24,10 @@ describe "String#to_c" do
     'NaN'.to_c.should == Complex(0, 0)
   end
 
-  it "understands a sequence of _" do
-    '7__9+4__0i'.to_c.should == Complex(79, 40)
+  ruby_bug "[Bug #19087]", ""..."3.2" do
+    it "disallows a sequence of _" do
+      '7__9+4__0i'.to_c.should == 7
+    end
   end
 
   it "allows null-byte" do