Use a slightly cleaner approach to stub ENV

spec/support/stub_env.rb

+# Inspired by https://github.com/ljkbennett/stub_env/blob/master/lib/stub_env/helpers.rb
 module StubENV
-  def stub_env(key, value)
-    allow(ENV).to receive(:[]).and_call_original unless @env_already_stubbed
-    @env_already_stubbed ||= true
+  def stub_env(key_or_hash, value = nil)
+    init_stub unless env_stubbed?
+    if key_or_hash.is_a? Hash
+      key_or_hash.each { |k, v| add_stubbed_value(k, v) }
+    else
+      add_stubbed_value key_or_hash, value
+    end
+  end
+
+  private
+
+  STUBBED_KEY = '__STUBBED__'.freeze
+
+  def add_stubbed_value(key, value)
     allow(ENV).to receive(:[]).with(key).and_return(value)
+    allow(ENV).to receive(:fetch).with(key).and_return(value)
+    allow(ENV).to receive(:fetch).with(key, anything()) do |_, default_val|
+      value || default_val
+    end
+  end
+
+  def env_stubbed?
+    ENV[STUBBED_KEY]
   end
-end
 
-# It's possible that the state of the class variables are not reset across
-# test runs.
-RSpec.configure do |config|
-  config.after(:each) do
-    @env_already_stubbed = nil
+  def init_stub
+    allow(ENV).to receive(:[]).and_call_original
+    allow(ENV).to receive(:fetch).and_call_original
+    add_stubbed_value(STUBBED_KEY, true)
   end
 end