Fix shared example; use #delete instead of #write

Author: Chris Hunt

spec/unit/github/auth/keys_file_spec.rb

@@ -159,7 +159,7 @@
       end
 
       it 'includes a newline after the last key' do
-        subject.write! keys
+        subject.delete! key
 
         expect(keys_file.read).to end_with("\n")
       end