Move message composition into checker classes

Having the MSG constant in the outer class, and the `#message` in the inner
class seemed like the class was split in the wrong location. Moving it into the
helper classes also means that we can check `RACK_LOADED` only once (in each
class) instead of twice.

lib/rubocop/cop/rspec/rails/http_status.rb

@@ -38,9 +38,6 @@ class HttpStatus < Cop
 
           include ConfigurableEnforcedStyle
 
-          MSG = 'Prefer `%<prefer>s` over `%<current>s` '\
-                'to describe HTTP status code.'.freeze
-
           def_node_matcher :http_status, <<-PATTERN
             (send nil? :have_http_status ${int sym})
           PATTERN
@@ -77,6 +74,11 @@ def checker_class
 
           # :nodoc:
           class SymbolicStyleChecker
+            MSG = 'Prefer `%<prefer>s` over `%<current>s` ' \
+                  'to describe HTTP status code.'.freeze
+            DEFAULT_MSG = 'Prefer `symbolic` over `numeric` ' \
+                  'to describe HTTP status code.'.freeze
+
             attr_reader :node
             def initialize(node)
               @node = node
@@ -87,19 +89,19 @@ def offensive?
             end
 
             def message
-              format(MSG, prefer: preferred_style, current: current_style)
+              if RACK_LOADED
+                format(MSG, prefer: preferred_style, current: number.to_s)
+              else
+                DEFAULT_MSG
+              end
             end
 
             def preferred_style
-              RACK_LOADED ? symbol.inspect : 'symbolic'
+              symbol.inspect
             end
 
             private
 
-            def current_style
-              RACK_LOADED ? number.to_s : 'numeric'
-            end
-
             def symbol
               ::Rack::Utils::SYMBOL_TO_STATUS_CODE.key(number)
             end
@@ -111,6 +113,11 @@ def number
 
           # :nodoc:
           class NumericStyleChecker
+            MSG = 'Prefer `%<prefer>s` over `%<current>s` ' \
+                  'to describe HTTP status code.'.freeze
+            DEFAULT_MSG = 'Prefer `numeric` over `symbolic` ' \
+                  'to describe HTTP status code.'.freeze
+
             WHITELIST_STATUS = %i[error success missing redirect].freeze
 
             attr_reader :node
@@ -123,19 +130,19 @@ def offensive?
             end
 
             def message
-              format(MSG, prefer: preferred_style, current: current_style)
+              if RACK_LOADED
+                format(MSG, prefer: preferred_style, current: symbol.inspect)
+              else
+                DEFAULT_MSG
+              end
             end
 
             def preferred_style
-              RACK_LOADED ? number.to_s : 'numeric'
+              number.to_s
             end
 
             private
 
-            def current_style
-              RACK_LOADED ? symbol.inspect : 'symbolic'
-            end
-
             def number
               ::Rack::Utils::SYMBOL_TO_STATUS_CODE[symbol]
             end