dont error on comparison of license with nil

lib/licensee/license.rb

@@ -115,7 +115,7 @@ class Licensee
     end
 
     def ==(other)
-      key == other.key
+      other != nil && key == other.key
     end
 
     private

lib/licensee/version.rb

@@ -1,3 +1,3 @@
 class Licensee
-  VERSION = "5.0.0b2"
+  VERSION = "5.0.0b3"
 end

test/test_licensee_license.rb

@@ -37,6 +37,7 @@ class TestLicenseeLicense < Minitest::Test
   should "know license equality" do
     assert @license == Licensee::License.new("MIT")
     refute @license == Licensee::License.new("ISC")
+    refute @license == nil
   end
 
   should "know if the license is featured" do