Allow to parse parameters with multiple values

Allow to receive multiple values for the parameter "file" for the
following source package commands: diff, linkdiff and servicediff.

src/api/app/controllers/source_controller.rb

@@ -13,6 +13,8 @@ class SourceController < ApplicationController
 
   skip_before_action :extract_user, only: [:lastevents_public, :global_command_orderkiwirepos, :global_command_triggerscmsync]
   skip_before_action :require_login, only: [:lastevents_public, :global_command_orderkiwirepos, :global_command_triggerscmsync]
+  # we use an array for the "file" parameter for: package_command_diff, package_command_linkdiff and package_command_servicediff
+  skip_before_action :validate_params, only: [:package_command]
 
   before_action :require_valid_project_name, except: [:index, :lastevents, :lastevents_public,
                                                       :global_command_orderkiwirepos, :global_command_branch,

src/api/app/lib/backend/connection.rb

@@ -77,9 +77,11 @@ def self.build_query_from_hash(hash, key_list = nil)
 
         if hash[key].nil?
           # just a boolean argument ?
-          [hash[key]].flat_map { key }.join('&')
+          key
+        elsif hash[key].is_a?(Array)
+          hash[key].map { |value| "#{key}=#{CGI.escape(value.to_s)}" }.join('&')
         else
-          [hash[key]].flat_map { "#{key}=#{CGI.escape(hash[key].to_s)}" }.join('&')
+          "#{key}=#{CGI.escape(hash[key].to_s)}"
         end
       end
       query.empty? ? '' : "?#{query.compact.join('&')}"