[core] choose encoded_description method content code earlier

Instead of runtime
see https://github.com/rspec/rspec-core/pull/2575/files#r227002134

---
This commit was imported from rspec/rspec-core@8372e5c.

rspec-core/lib/rspec/core/formatters/exception_presenter.rb

@@ -244,12 +244,13 @@ def formatted_message_and_backtrace(colorizer)
           end
         end
 
-        def encoded_description(description)
-          return if description.nil?
-
-          if String.method_defined?(:encoding)
+        if  String.method_defined?(:encoding)
+          def encoded_description(description)
+            return if description.nil?
             encoded_string(description)
-          else # for 1.8.7
+          end
+        else # for 1.8.7
+          def encoded_description(description)
             description
           end
         end