Add RSpec respond_to matcher spec

spec/rspec/expectations/respond_to_matcher_spec.cr

@@ -0,0 +1,29 @@
+require "../../spec_helper"
+
+# Examples taken from:
+# https://relishapp.com/rspec/rspec-expectations/v/3-8/docs/built-in-matchers/respond-to-matcher
+# and modified to fit Spectator and Crystal.
+Spectator.describe "`respond_to` matcher" do
+  context "basic usage" do
+    describe "a string" do
+      it { is_expected.to respond_to(:size) } # It's size in Crystal, not length.
+      it { is_expected.to respond_to(:hash, :class, :to_s) }
+      it { is_expected.not_to respond_to(:to_model) }
+      it { is_expected.not_to respond_to(:compact, :flatten) }
+
+      # deliberate failures
+      # TODO: Add support for expected failures.
+      xit { is_expected.to respond_to(:to_model) }
+      xit { is_expected.to respond_to(:compact, :flatten) }
+      xit { is_expected.not_to respond_to(:size) }
+      xit { is_expected.not_to respond_to(:hash, :class, :to_s) }
+
+      # mixed examples--String responds to :length but not :flatten
+      # both specs should fail
+      xit { is_expected.to respond_to(:size, :flatten) }
+      xit { is_expected.not_to respond_to(:size, :flatten) }
+    end
+  end
+
+  # Spectator doesn't support argument matching with respond_to.
+end