[ruby/base64] Avoid unnecessary string duplication

String#ljust returns a new string, so whenever we need to add
padding, we can replace "-/" in place with String#tr! and avoid creating
yet another copy of the string.

https://github.com/ruby/base64/commit/6401ef5824

lib/base64.rb

@@ -99,9 +99,11 @@ module Base64
     # NOTE: RFC 4648 does say nothing about unpadded input, but says that
     # "the excess pad characters MAY also be ignored", so it is inferred that
     # unpadded input is also acceptable.
-    str = str.tr("-_", "+/")
     if !str.end_with?("=") && str.length % 4 != 0
       str = str.ljust((str.length + 3) & ~3, "=")
+      str.tr!("-_", "+/")
+    else
+      str = str.tr("-_", "+/")
     end
     strict_decode64(str)
   end