[ruby/base64] Improve Base64.urlsafe_encode64 performance

Improves the method's performance when asked to remove padding.

str.delete!("=") iterates over the entire string looking for the equals
character, but we know that we will, at most, find two at the end of the
string.

https://github.com/ruby/base64/commit/544e0c2cf7

lib/base64.rb

@@ -82,8 +82,14 @@ module Base64
   # You can remove the padding by setting +padding+ as false.
   def urlsafe_encode64(bin, padding: true)
     str = strict_encode64(bin)
+    unless padding
+      if str.end_with?("==")
+        str.delete_suffix!("==")
+      elsif str.end_with?("=")
+        str.chop!
+      end
+    end
     str.tr!("+/", "-_")
-    str.delete!("=") unless padding
     str
   end