Check that actual value is a collection

Previously, a compilation error would occur if the actual value didn't 
respond to `to_a`.
A way to reproduce this is for the actual value to be nilable.

src/spectator/matchers/array_matcher.cr

@@ -23,7 +23,10 @@ module Spectator::Matchers
 
     # Actually performs the test against the expression.
     def match(actual : TestExpression(T)) : MatchData forall T
-      actual_elements = actual.value.to_a
+      actual_value = actual.value
+      return unexpected(actual_value, actual.label) unless actual_value.responds_to?(:to_a)
+
+      actual_elements = actual_value.to_a
       expected_elements = expected.value.to_a
       missing, extra = compare_arrays(expected_elements, actual_elements)
 
@@ -44,7 +47,10 @@ module Spectator::Matchers
     # Performs the test against the expression, but inverted.
     # A successful match with `#match` should normally fail for this method, and vice-versa.
     def negated_match(actual : TestExpression(T)) : MatchData forall T
-      actual_elements = actual.value.to_a
+      actual_value = actual.value
+      return unexpected(actual_value, actual.label) unless actual_value.responds_to?(:to_a)
+
+      actual_elements = actual_value.to_a
       expected_elements = expected.value.to_a
       missing, extra = compare_arrays(expected_elements, actual_elements)
 
@@ -108,5 +114,9 @@ module Spectator::Matchers
         Array.new(count, element)
       end.flatten
     end
+
+    private def unexpected(value, label)
+      raise "#{label} is not a collection (must respond to `#to_a`). #{label}: #{value.inspect}"
+    end
   end
 end

src/spectator/matchers/unordered_array_matcher.cr

@@ -20,7 +20,10 @@ module Spectator::Matchers
 
     # Actually performs the test against the expression.
     def match(actual : TestExpression(T)) : MatchData forall T
-      actual_elements = actual.value.to_a
+      actual_value = actual.value
+      return unexpected(actual_value, actual.label) unless actual_value.responds_to?(:to_a)
+
+      actual_elements = actual_value.to_a
       expected_elements = expected.value.to_a
       missing, extra = array_diff(expected_elements, actual_elements)
 
@@ -39,7 +42,10 @@ module Spectator::Matchers
     # Performs the test against the expression, but inverted.
     # A successful match with `#match` should normally fail for this method, and vice-versa.
     def negated_match(actual : TestExpression(T)) : MatchData forall T
-      actual_elements = actual.value.to_a
+      actual_value = actual.value
+      return unexpected(actual_value, actual.label) unless actual_value.responds_to?(:to_a)
+
+      actual_elements = actual_value.to_a
       expected_elements = expected.value.to_a
       missing, extra = array_diff(expected_elements, actual_elements)
 
@@ -71,5 +77,9 @@ module Spectator::Matchers
 
       {missing, extra}
     end
+
+    private def unexpected(value, label)
+      raise "#{label} is not a collection (must respond to `#to_a`). #{label}: #{value.inspect}"
+    end
   end
 end