Fix RuntimeError in Lograge::Formatters::Graylog2

Rubocop auto-corrected `data_clone.each.keys` to `data_clone.each_key`
which resulted in the following error:

     RuntimeError:
       can't add a new key into hash during iteration

To fix this, we use Hash#transform_keys (introduced in Ruby 2.5.0).

lib/lograge/formatters/graylog2.rb

@@ -6,20 +6,10 @@ class Graylog2
       include Lograge::Formatters::Helpers::MethodAndPath
 
       def call(data)
-        # Cloning because we don't want to mess with the original when mutating keys.
-        data_clone = data.clone
-
-        base = {
-          short_message: short_message(data_clone)
-        }
-
         # Add underscore to every key to follow GELF additional field syntax.
-        data_clone.each_key do |key|
-          data_clone[underscore_prefix(key)] = data_clone[key]
-          data_clone.delete(key)
-        end
-
-        data_clone.merge(base)
+        data.transform_keys { |k| underscore_prefix(k) }.merge(
+          short_message: short_message(data)
+        )
       end
 
       def underscore_prefix(key)