no more is_a checks on instantiation

actionpack/lib/action_dispatch/journey/path/pattern.rb

@@ -11,15 +11,10 @@ def self.from_string string
         end
 
         def initialize(strexp)
-          case strexp
-          when Router::Strexp
-            @spec         = strexp.ast
-            @requirements = strexp.requirements
-            @separators   = strexp.separators.join
-            @anchored     = strexp.anchor
-          else
-            raise ArgumentError, "Bad expression: #{strexp}"
-          end
+          @spec         = strexp.ast
+          @requirements = strexp.requirements
+          @separators   = strexp.separators.join
+          @anchored     = strexp.anchor
 
           @names          = nil
           @optional_names = nil

actionpack/test/journey/path/pattern_test.rb

@@ -75,10 +75,6 @@ class TestPattern < ActiveSupport::TestCase
           end
         end
 
-        def test_to_raise_exception_with_bad_expression
-          assert_raise(ArgumentError, "Bad expression: []") { Pattern.new [] }
-        end
-
         def test_to_regexp_with_extended_group
           strexp = Router::Strexp.build(
             '/page/:name',