Update quotes for heredocs with embedded Ruby with \n

The default behaviour for heredocs is the double quote style of quotes, which
means the generated code looks like:

    ObjectSpace.define_finalizer(obj, Proc.new { STDOUT.write "finalized
    " })

It works, but it's ugly.

core/binding/shared/clone.rb

@@ -40,7 +40,7 @@
     end
 
     it "copies the finalizer" do
-      code = <<-RUBY
+      code = <<-'RUBY'
         obj = binding
 
         ObjectSpace.define_finalizer(obj, Proc.new { STDOUT.write "finalized\n" })

core/method/shared/dup.rb

@@ -16,7 +16,7 @@
     end
 
     it "copies the finalizer" do
-      code = <<-RUBY
+      code = <<-'RUBY'
         obj = Object.new.method(:method)
 
         ObjectSpace.define_finalizer(obj, Proc.new { STDOUT.write "finalized\n" })

core/objectspace/define_finalizer_spec.rb

@@ -156,7 +156,7 @@ def finalizer(zelf)
   end
 
   it "allows multiple finalizers with different 'callables' to be defined" do
-    code = <<-RUBY
+    code = <<-'RUBY'
       obj = Object.new
 
       ObjectSpace.define_finalizer(obj, Proc.new { STDOUT.write "finalized1\n" })

core/proc/shared/dup.rb

@@ -25,7 +25,7 @@
     end
 
     it "copies the finalizer" do
-      code = <<-RUBY
+      code = <<-'RUBY'
         obj = Proc.new { }
 
         ObjectSpace.define_finalizer(obj, Proc.new { STDOUT.write "finalized\n" })

core/unboundmethod/shared/dup.rb

@@ -16,7 +16,7 @@
     end
 
     it "copies the finalizer" do
-      code = <<-RUBY
+      code = <<-'RUBY'
         obj = Class.instance_method(:instance_method)
 
         ObjectSpace.define_finalizer(obj, Proc.new { STDOUT.write "finalized\n" })