Stub ENVs in RSpec.

rspec_stub_env.rb

def stub_env(key, value)
  is_set_up_flag = "__stub_env_is_set_up"

  unless ENV[is_set_up_flag]
    allow(ENV).to receive(:[]).and_call_original
    allow(ENV).to receive(:fetch).and_call_original

    allow(ENV).to receive(:[]).with(is_set_up_flag).and_return(true)
  end

  allow(ENV).to receive(:fetch).with(key).and_return(value)
  allow(ENV).to receive(:[]).with(key).and_return(value)
end