Options: add error class, refactor

- improve readability of specs
  - extract methods

spec/unit/options_spec.rb

@@ -1,16 +1,42 @@
 # frozen_string_literal: true
-describe GitHubChangelogGenerator::Options do
-  it 'can be instantiated with defaults' do
-    expect(described_class.new(user: 'olle')[:user]).to eq('olle')
+RSpec.describe GitHubChangelogGenerator::Options do
+  describe "#initialize" do
+    context "with known options" do
+      it "instantiates successfully" do
+        expect(described_class.new(user: "olle")[:user]).to eq("olle")
+      end
+    end
+
+    context "with unknown options" do
+      it "raises an error" do
+        expect {
+          described_class.new(
+            git_remote: "origin",
+            strength: "super-strength",
+            wisdom: "deep"
+          )
+        }.to raise_error("[:strength, :wisdom]")
+      end
+    end
   end
 
-  it 'disallows unknown option names' do
-    expect {
-      described_class.new(
-        git_remote: 'origin',
-        strength: 'super-strength',
-        wisdom: 'deep'
-      )
-    }.to raise_error(ArgumentError, "Unsupported options [:strength, :wisdom]")
+  describe "#[]=(key, value)" do
+    let(:options) { described_class.new(git_remote: "origin") }
+
+    context "with known options" do
+      it "sets the option value" do
+        expect {
+          options[:git_remote] = "in the cloud"
+        }.to change { options[:git_remote] }.to "in the cloud"
+      end
+    end
+
+    context "with unknown options" do
+      it "raises an error" do
+        expect {
+          options[:charisma] = 8
+        }.to raise_error(":charisma")
+      end
+    end
   end
 end