Redact short and known private tokens

Prior this commit very short private tokens (< 4 chars) triggered:
ArgumentError: negative argument

Also, if private token was a term which was part of the inspected string
only the first occurrence was redacted.

lib/gitlab/client.rb

@@ -77,7 +77,7 @@ class Client < API
     # @return [String]
     def inspect
       inspected = super
-      inspected.sub! @private_token, only_show_last_four_chars(@private_token) if @private_token
+      inspected = redact_private_token(inspected, @private_token) if @private_token
       inspected
     end
 
@@ -91,7 +91,14 @@ def url_encode(url)
 
     private
 
+    def redact_private_token(inspected, private_token)
+      redacted = only_show_last_four_chars(private_token)
+      inspected.sub %(@private_token="#{private_token}"), %(@private_token="#{redacted}")
+    end
+
     def only_show_last_four_chars(token)
+      return '****' if token.size <= 4
+
       "#{'*' * (token.size - 4)}#{token[-4..]}"
     end
   end

spec/gitlab/client_spec.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Client do
+  subject(:client) { described_class.new(options) }
+
+  describe '#inspect' do
+    subject { client.inspect }
+
+    context 'without private token' do
+      let(:options) { { private_token: nil } }
+
+      it { is_expected.not_to include('@private_token=') }
+    end
+
+    context 'with a some lengthy private token' do
+      let(:options) { { private_token: 'some token' } }
+
+      it { is_expected.to include('@private_token="******oken"') }
+    end
+
+    context 'with a known private token' do
+      let(:options) { { private_token: 'endpoint' } }
+
+      it { is_expected.to include('@private_token="****oint"') }
+      it { is_expected.to include('@endpoint=') }
+    end
+
+    context 'with empty private token' do
+      let(:options) { { private_token: '' } }
+
+      it { is_expected.to include('@private_token="****"') }
+    end
+
+    context 'with short private token' do
+      let(:options) { { private_token: 'abcd' } }
+
+      it { is_expected.to include('@private_token="****"') }
+    end
+  end
+end