Return frozen strings for nil/true/false to_s

Author: bjfish

spec/tags/core/false/to_s_tags.txt

@@ -29,7 +29,7 @@
 class FalseClass
 
   def to_s
-    +'false'
+    'false'
   end
 
   def inspect

spec/tags/core/nil/to_s_tags.txt

@@ -40,7 +40,7 @@ def ^(other)
   end
 
   def to_s
-    +''
+    ''
   end
 
   def inspect

spec/tags/core/true/to_s_tags.txt

@@ -29,7 +29,7 @@
 class TrueClass
 
   def to_s
-    +'true'
+    'true'
   end
 
   def inspect