[PodTargetInstaller] Properly set the attributes of private and project headers

lib/cocoapods/installer/target_installer/pod_target_installer.rb

@@ -50,6 +50,7 @@ module Pod
 
           headers = file_accessor.headers
           public_headers = file_accessor.public_headers
+          private_headers = file_accessor.private_headers
           other_source_files = file_accessor.source_files.select { |sf| sf.extname == '.d' }
 
           {
@@ -66,9 +67,13 @@ module Pod
           native_target.add_file_references(header_file_refs) do |build_file|
             # Set added headers as public if needed
             if target.requires_frameworks?
+              build_file.settings ||= {}
               if public_headers.include?(build_file.file_ref.real_path)
-                build_file.settings ||= {}
                 build_file.settings['ATTRIBUTES'] = ['Public']
+              elsif private_headers.include?(build_file.file_ref.real_path)
+                build_file.settings['ATTRIBUTES'] = ['Private']
+              else
+                build_file.settings['ATTRIBUTES'] = ['Project']
               end
             end
           end

lib/cocoapods/sandbox/file_accessor.rb

@@ -124,6 +124,12 @@ module Pod
         header_files - private_headers
       end
 
+      # @return [Array<Pathname>] The private headers of the specification.
+      #
+      def private_headers
+        private_header_files
+      end
+
       # @return [Hash{ Symbol => Array<Pathname> }] the resources of the
       #         specification grouped by destination.
       #