Make Angle immutable

Author: rhannequin

lib/astronoby/angle.rb

@@ -70,6 +70,7 @@ def initialize(radians)
       else
         BigDecimal(radians, PRECISION)
       end
+      freeze
     end
 
     def degrees

spec/astronoby/angle_spec.rb

@@ -1,6 +1,20 @@
 # frozen_string_literal: true
 
 RSpec.describe Astronoby::Angle do
+  describe "object immutability" do
+    context "when a mutable method is added" do
+      it "raises a FrozenError when immutability is broken" do
+        described_class.class_eval do
+          def clear
+            @radians = 0
+          end
+        end
+
+        expect { described_class.as_degrees(180).clear }.to raise_error(FrozenError)
+      end
+    end
+  end
+
   describe "::as_radians" do
     it "returns an Angle object" do
       expect(described_class.as_radians(described_class::PI))