Make it harder to get at the secret in Generator/Verifier

This is protecting against a developer being careless
about introspection and stuff winding up in debug logs.

The implementation involved making attr_readers on :secrets
private, and overriding to_s and inspect so that they don't
show up if people print them out and log output.

lib/fernet/generator.rb

@@ -5,7 +5,6 @@
 
 module Fernet
   class Generator
-    attr_reader :secret
     attr_accessor :data
 
     def initialize(secret)
@@ -20,5 +19,12 @@ def generate
       Base64.urlsafe_encode64(JSON.dump(data.merge(signature: mac)))
     end
 
+    def inspect
+      "#<Fernet::Generator @secret=[masked] @data=#{@data.inspect}>"
+    end
+    alias to_s inspect
+
+  private
+    attr_reader :secret
   end
 end

lib/fernet/verifier.rb

@@ -6,11 +6,11 @@
 module Fernet
   class Verifier
 
-    attr_reader :secret, :token, :data
+    attr_reader :token, :data
     attr_writer :seconds_valid
 
     def initialize(secret)
-      @secret        = secret
+      @secret = secret
     end
 
     def verify_token(token)
@@ -22,7 +22,13 @@ def verify_token(token)
       signatures_match? && token_recent_enough? && custom_verification
     end
 
-    private
+    def inspect
+      "#<Fernet::Verifier @secret=[masked] @token=#{@token} @data=#{@data.inspect} @seconds_valid=#{@seconds_valid}>"
+    end
+    alias to_s inspect
+
+  private
+    attr_reader :secret
 
     def deconstruct
       @data               = JSON.parse(Base64.decode64(token))