Change Relation#load to match AR implementation

`Relation#load` method:
- Accepts a block as an argument
- Checks also for `scheduled?` predicate
- Returns the `Relation` itself (not an array)

This signature is available from 7.0.0, the minimum requested AR version

Close #297

Ref: rails/rails@caa178c

lib/chrono_model/patches/relation.rb

@@ -23,10 +23,12 @@ def empty_scope?
         @values == klass.unscoped.as_of(as_of_time).values
       end
 
-      def load
-        return super unless @_as_of_time && !loaded?
+      def load(&block)
+        super
 
-        super.each { |record| record.as_of_time!(@_as_of_time) }
+        @records.each { |record| record.as_of_time!(@_as_of_time) } if @_as_of_time
+
+        self
       end
 
       def merge(*)