Better handling with be match against Value types and nil

Releates to https://github.com/icy-arctic-fox/spectator/issues/37

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Fixed
 - Fix usage of `be ===` and `be =~` [#34](https://github.com/icy-arctic-fox/spectator/issues/34)
+- Better handling of the `be(nil)` when used with value types. [#37](https://github.com/icy-arctic-fox/spectator/issues/37)
 
 ### Changed
 - Elegantly handle missing/undefined methods with `have_attributes` matcher.

src/spectator/matchers/reference_matcher.cr

@@ -14,7 +14,9 @@ module Spectator::Matchers
     # Checks whether the matcher is satisifed with the expression given to it.
     private def match?(actual : Expression(T)) : Bool forall T
       value = expected.value
-      if value && value.responds_to?(:same?)
+      if value.nil?
+        actual.value.nil?
+      elsif value.responds_to?(:same?)
         value.same?(actual.value)
       else
         # Value type (struct) comparison.