FIX: don't break the private key when writing it out during theme import

* when importing a private theme using the themes:install rake task the SSH key is written out to a file for use by the git-clone command
* if the private key is written out without a newline at end-of-file (i.e. after it's been stripped) it's not recognized as a valid key by SSH
* so: don't strip it when writing it out, we should be fine

lib/theme_store/git_importer.rb

@@ -107,7 +107,7 @@ class ThemeStore::GitImporter
     ssh_folder = "#{Pathname.new(Dir.tmpdir).realpath}/discourse_theme_ssh_#{SecureRandom.hex}"
     FileUtils.mkdir_p ssh_folder
 
-    File.write("#{ssh_folder}/id_rsa", @private_key.strip)
+    File.write("#{ssh_folder}/id_rsa", @private_key)
     FileUtils.chmod(0600, "#{ssh_folder}/id_rsa")
 
     begin