Handle undefined methods with have_attributes matcher

Instead of producing a compilation error on missing/undefind methods, 
substitute in an "undefined" value.
This shows better output to the user.

CHANGELOG.md

@@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - Fix usage of `be ===` and `be =~` [#34](https://github.com/icy-arctic-fox/spectator/issues/34)
 
+### Changed
+- Elegantly handle missing/undefined methods with `have_attributes` matcher.
+
 ## [0.10.1] - 2021-09-16
 ### Fixed
 - Fix `Spectator.configure` block calls to `filter_run_excluding` and `filter_run_including`. [#61](https://gitlab.com/arctic-fox/spectator/-/issues/61)

src/spectator/matchers/attributes_matcher.cr

@@ -10,6 +10,16 @@ module Spectator::Matchers
   # Each key in the tuple is the attribute/method name,
   # and the corresponding value is the expected value to match against.
   struct AttributesMatcher(ExpectedType) < Matcher
+    # Stand-in for undefined methods on types.
+    private module Undefined
+      extend self
+
+      # Text displayed when a method is undefined.
+      def inspect(io)
+        io << "<Method undefined>"
+      end
+    end
+
     # Expected value and label.
     private getter expected
 
@@ -51,7 +61,7 @@ module Spectator::Matchers
       {% begin %}
       {
         {% for attribute in ExpectedType.keys %}
-        {{attribute}}: object.{{attribute}},
+        {{attribute}}: object.responds_to?({{attribute.symbolize}}) ? object.{{attribute}} : Undefined,
         {% end %}
       }
       {% end %}