Consolidate set_session and set_flash APIs

* Refactor so they both use SetSessionOrFlashMatcher internally
* Remove `set_session['key']` in favor of `set_session('key')`
* `set_flash['key'].to(nil)` no longer works if set_flash has never been
  set

lib/shoulda/matchers/action_controller/flash_store.rb

@@ -0,0 +1,95 @@
+module Shoulda
+  module Matchers
+    module ActionController
+      # @private
+      class FlashStore
+        def self.future
+          new
+        end
+
+        def self.now
+          new.use_now!
+        end
+
+        attr_accessor :controller
+
+        def initialize
+          @use_now = false
+        end
+
+        def name
+          if @use_now
+            'flash.now'
+          else
+            'flash'
+          end
+        end
+
+        def has_key?(key)
+          values_to_check.include?(key)
+        end
+
+        def has_value?(expected_value)
+          values_to_check.values.any? do |actual_value|
+            expected_value === actual_value
+          end
+        end
+
+        def empty?
+          flash.empty?
+        end
+
+        def use_now!
+          @use_now = true
+          self
+        end
+
+        private
+
+        def flash
+          @_flash ||= copy_of_flash_from_controller
+        end
+
+        def copy_of_flash_from_controller
+          controller.flash.dup.tap do |flash|
+            copy_flashes(controller.flash, flash)
+            copy_discard_if_necessary(controller.flash, flash)
+            # sweep_flash_if_necessary(flash)
+          end
+        end
+
+        def copy_flashes(original_flash, new_flash)
+          flashes = original_flash.instance_variable_get('@flashes').dup
+          new_flash.instance_variable_set('@flashes', flashes)
+        end
+
+        def copy_discard_if_necessary(original_flash, new_flash)
+          discard = original_flash.instance_variable_get('@discard').dup
+          new_flash.instance_variable_set('@discard', discard)
+        end
+
+        def sweep_flash_if_necessary(flash)
+          unless @use_now
+            flash.sweep
+          end
+        end
+
+        def set_values
+          flash.instance_variable_get('@flashes')
+        end
+
+        def keys_to_discard
+          flash.instance_variable_get('@discard')
+        end
+
+        def values_to_check
+          if @use_now
+            set_values.slice(*keys_to_discard.to_a)
+          else
+            set_values.except(*keys_to_discard.to_a)
+          end
+        end
+      end
+    end
+  end
+end