Always cast enummer values to symbols

Support for using strings as setters was added in 441cdf6, but the
initial implementation suffers from an idempotency issue, as changes
would be committed to the DB even if the values were the same. Consider
the following scenario:

    u = User.last
    => #<User:0x00007c747d137810 id: 3, permissions: [:read, :write], facial_features: [], diets: [], transport: [], home: []>

    u.permissions = u.permissions.map(&:to_s)
    => ["read", "write"]

    u.changes
    => {"permissions"=>[[:read, :write], ["read", "write"]]}

Because we cast the values to symbols when serializing, we set the
correct bitmask but this comes with the drawback of altering the DB
even if the new values are the same.

A better way to handle this issue is to delegate the responsibility to
the `cast` method. If we do this we can drop the initial fix, as values
will _always_ be symbols.

It's tricky to test something like this, so I've opted to hook into
ActiveRecord notifications and confirm no SQL UPDATE is being triggered
if the values are the same.

lib/enummer/enummer_type.rb

@@ -21,7 +21,7 @@ def type
     def serialize(value)
       return unless value
 
-      Array.wrap(value).sum { |value_name| @values.fetch(value_name.to_sym, 0) }
+      Array.wrap(value).sum { |value_name| @values.fetch(value_name, 0) }
     end
 
     # @param [Numeric] value Numeric representation of values
@@ -36,5 +36,11 @@ def deserialize(value)
         value_names << pair_name
       end
     end
+
+    # @param [Array<Symbol>] value Current value represented as one or more symbols or strings
+    # @return [Array<Symbol>] Current value represented as symbols
+    def cast(value)
+      Array.wrap(value).map(&:to_sym)
+    end
   end
 end

test/enummer_test.rb

@@ -77,9 +77,18 @@ def setup
   end
 
   test "setting the attribute with strings adds the values" do
-    @user3.update(permissions: ["read", "write"])
+    @user3.update(permissions: %w[read write])
 
     assert_equal %i[read write], @user3.permissions
+
+    updated = false
+    callback = lambda { |_name, _start, _finish, _id, payload| updated = true if payload[:sql].starts_with?("UPDATE") }
+
+    ActiveSupport::Notifications.subscribed(callback, "sql.active_record") do
+      @user3.update(permissions: %w[read write])
+    end
+
+    refute updated, "subsequent updates with the same values should be idempotent"
   end
 
   test "using a bang method properly updates the persisted field" do