Add spec for empty matcher

Mark question matchers as done (for now).
More may be added later if they're common.

README.md

@@ -114,7 +114,7 @@ In no particular order, features that have been implemented and are planned:
     - [X] Equality matchers - `eq`, `be`, `be_a`, `match`
     - [X] Truthy matchers - `be_true`, `be_false`, `be_truthy`, `be_falsey`
     - [X] Comparison matchers - `<`, `<=`, `>`, `>=`, `be_within`
-    - [ ] Question matchers - `be_nil`, `be_xxx`
+    - [X] Question matchers - `be_nil`, `be_empty`
     - [ ] Exception matchers - `raise_error`
     - [ ] Collection matchers - `start_with`, `end_with`, `contain`, `contain_exactly`
     - [ ] Change matchers - `change`, `from`, `to`, `by`, `by_at_least`, `by_at_most`

spec/matchers/empty_matcher_spec.cr

@@ -0,0 +1,43 @@
+require "../spec_helper"
+
+describe Spectator::Matchers::EmptyMatcher do
+  describe "#match?" do
+    context "with an empty set" do
+      it "is true" do
+        array = [] of Symbol
+        partial = Spectator::Expectations::ValueExpectationPartial.new(array)
+        matcher = Spectator::Matchers::EmptyMatcher.new
+        matcher.match?(partial).should be_true
+      end
+    end
+
+    context "with a filled set" do
+      it "is false" do
+        array = %i[a b c]
+        partial = Spectator::Expectations::ValueExpectationPartial.new(array)
+        matcher = Spectator::Matchers::EmptyMatcher.new
+        matcher.match?(partial).should be_false
+      end
+    end
+  end
+
+  describe "#message" do
+    it "contains the actual label" do
+      array = %i[a b c]
+      label = "everything"
+      partial = Spectator::Expectations::ValueExpectationPartial.new(label, array)
+      matcher = Spectator::Matchers::EmptyMatcher.new
+      matcher.message(partial).should contain(label)
+    end
+  end
+
+  describe "#negated_message" do
+    it "contains the actual label" do
+      array = %i[a b c]
+      label = "everything"
+      partial = Spectator::Expectations::ValueExpectationPartial.new(label, array)
+      matcher = Spectator::Matchers::EmptyMatcher.new
+      matcher.negated_message(partial).should contain(label)
+    end
+  end
+end