Support variables and methods for type matcher (be_a).

Addresses https://github.com/icy-arctic-fox/spectator/issues/25

spec/matchers/type_matcher_spec.cr

@@ -0,0 +1,26 @@
+require "../spec_helper"
+
+Spectator.describe Spectator::Matchers::TypeMatcher do
+  context String do # Sets `described_class` to String
+    def other_type
+      Int32
+    end
+
+    describe "#|" do
+      it "works on sets" do
+        super_set = (described_class | other_type)
+
+        expect(42).to be_kind_of(super_set)
+        expect("foo").to be_a(super_set)
+      end
+    end
+
+    it "works on described_class" do
+      expect("foo").to be_a_kind_of(described_class)
+    end
+
+    it "works on plain types" do
+      expect(42).to be_a(Int32)
+    end
+  end
+end