fixes attachment download path in api

Kudos to @oliverguenter for the fix

doc/apiv3-documentation.apib

@@ -751,7 +751,7 @@ container (e.g. a work package or a board message).
                         "href": "/api/v3/users/1"
                     },
                     "downloadLocation": {
-                        "href": "/attachments/1"
+                        "href": "/attachments/1/download"
                     }
                 },
                 "id": 1,
@@ -875,7 +875,7 @@ Permanently deletes the specified attachment.
                                     "href": "/api/v3/users/1"
                                 },
                                 "downloadLocation": {
-                                    "href": "/attachments/1"
+                                    "href": "/attachments/1/download"
                                 }
                             },
                             "id": 1,
@@ -906,7 +906,7 @@ Permanently deletes the specified attachment.
                                     "href": "/api/v3/users/1"
                                 },
                                 "downloadLocation": {
-                                    "href": "/attachments/2"
+                                    "href": "/attachments/2/download"
                                 }
                             },
                             "id": 2,

lib/api/v3/utilities/path_helper.rb

@@ -49,7 +49,7 @@ module API
           end
 
           def self.attachment_download(id)
-            attachment_path(id)
+            download_attachment_path(id)
           end
 
           def self.attachments_by_work_package(id)

spec/lib/api/v3/utilities/path_helper_spec.rb

@@ -69,7 +69,7 @@ describe ::API::V3::Utilities::PathHelper do
   describe '#attachment_download' do
     subject { helper.attachment_download 1 }
 
-    it_behaves_like 'path', '/attachments/1'
+    it_behaves_like 'path', '/attachments/1/download'
   end
 
   describe '#attachments_by_work_package' do