add tests for fetching token

spec/unit/fetcher_spec.rb

@@ -0,0 +1,19 @@
+describe GitHubChangelogGenerator::Fetcher do
+  before(:all) do
+    @fetcher = GitHubChangelogGenerator::Fetcher.new
+  end
+
+  describe "#fetch_github_token" do
+    token = GitHubChangelogGenerator::Fetcher::CHANGELOG_GITHUB_TOKEN
+    context "when token in ENV exist" do
+      before { stub_const("ENV", ENV.to_hash.merge(token => "0123456789abcdef")) }
+      subject { @fetcher.fetch_github_token }
+      it { is_expected.to eq("0123456789abcdef") }
+    end
+    context "when token in ENV is nil" do
+      before { stub_const("ENV", ENV.to_hash.merge(token => nil)) }
+      subject { @fetcher.fetch_github_token }
+      it { is_expected.to be_nil }
+    end
+  end
+end