Fix Performance/StringInclude that RuboCop can't

This replaces two instances of /html/.match?(<string>) with the more
performant <string>.include?("html").

Performance/StringInclude was enabled in 3158bbb, however RuboCop does
not flag these two because it is unable to determine that the variable
passed to #match? is a string. In both these cases we know that the
variable must be a string (Mime::Type must be initialized with a string,
and Content-Type must be a string if present per Rack SPEC)

These were found by `rg '/\[\w ]+/\.match\?'`, and while this search
returns other entries they are either comments or in test files.

actionpack/lib/action_dispatch/http/mime_type.rb

@@ -293,7 +293,7 @@ module Mime
     end
 
     def html?
-      (symbol == :html) || /html/.match?(@string)
+      (symbol == :html) || @string.include?("html")
     end
 
     def all?; false; end

actionpack/lib/action_dispatch/http/permissions_policy.rb

@@ -55,7 +55,7 @@ module ActionDispatch # :nodoc:
       private
         def html_response?(headers)
           if content_type = headers[CONTENT_TYPE]
-            /html/.match?(content_type)
+            content_type.include?("html")
           end
         end