Base: read from known_attributes in respond_to_missing?

Prior to this commit, the `respond_to_missing?` implementation had a
conditional to supports attributes being able to be written to through
methods "did not respond to" (like `send("#{attr_name}=", value)`), as
well as `?`-suffixed predicate methods.

The `method_missing` implementation has a set of conditionals similar to
`respond_to_missing`, but with subtle variations through checking for an
attribute name's inclusion in `known_attributes` (attributes defined by
the Schema **or** assigned during `Base#load`) rather than the attribute
name's inclusion in `attributes` (the current instance's assigned
values, without any attributes declared in the Schema).

Without the implementation changes, the following tests introduced in
this commit fail:

```
  1) Failure:
BaseTest#test_respond_to_known_attributes [test/cases/base_test.rb:972]:
Expected #<Person:0x000000011d0cdcf8 @attributes={}, @prefix_options={}, @persisted=false> (Person) to respond to #name=.
```

This commit changes the `respond_to_missing?` conditional to check for
the presence of the key in `known_attributes` rather than `attributes`
so that instances will respond to `=`-suffixed writers and `?`-suffixed
predicates for attributes that are declared in the resource's Schema
that have not yet been assigned to.

Author: seanpdoyle

lib/active_resource/base.rb

@@ -1653,7 +1653,7 @@ def respond_to_missing?(method, include_priv = false)
         super
       elsif known_attributes.include?(method_name)
         true
-      elsif method_name =~ /(?:=|\?)$/ && attributes.include?($`)
+      elsif method_name =~ /(?:=|\?)$/ && known_attributes.include?($`)
         true
       else
         # super must be called at the end of the method, because the inherited respond_to?

test/cases/base_test.rb

@@ -960,7 +960,52 @@ def test_respond_to
     assert_respond_to matz, :name
     assert_respond_to matz, :name=
     assert_respond_to matz, :name?
-    assert_not matz.respond_to?(:super_scalable_stuff)
+    assert_not_respond_to matz, :super_scalable_stuff
+    assert_not_respond_to matz, :super_scalable_stuff=
+  end
+
+  def test_respond_to_known_attributes
+    previous_schema = Person.schema
+    Person.schema = { name: "string" }
+
+    person = Person.new
+
+    assert_respond_to person, :name
+    assert_respond_to person, :name=
+    assert_not_respond_to person, :super_scalable_stuff
+    assert_not_respond_to person, :super_scalable_stuff=
+  ensure
+    Person.schema = previous_schema
+  end
+
+  def test_reading_an_unknown_attribute_raises_NoMethodError
+    assert_raises NoMethodError, match: "unknown_attribute" do
+      Post.new.unknown_attribute
+    end
+  end
+
+  def test_writing_an_unknown_attribute_assigns_a_value_that_can_be_read
+    post = Post.new
+
+    post.unknown_attribute = "assigned"
+
+    assert_respond_to post, :unknown_attribute
+    assert_equal "assigned", post.unknown_attribute
+  end
+
+  def test_writing_nil_to_an_existing_attribute_can_be_read
+    post = Post.new unknown_attribute: "assigned"
+
+    post.unknown_attribute = nil
+
+    assert_nil post.unknown_attribute
+    assert_respond_to post, :unknown_attribute
+  end
+
+  def test_predicate_for_an_unknown_attribute_returns_nil
+    post = Post.new
+
+    assert_not_predicate post, :unknown_attribute?
   end
 
   def test_custom_header