Add missing expectation before checking metadata

We weren't checking whether the request creating the proposal had
finished before checking the document. That's probably why this test has
recently failed on our CI:

```
1) Documents Metadata download document without metadata
     Failure/Error: io = URI.parse(
                          "#{app_host}#{polymorphic_path(
                                        Document.last.attachment)}"
                        ).open

     NoMethodError:
       undefined method `attachment' for nil:NilClass
```

spec/system/documents_spec.rb

@@ -11,9 +11,10 @@ describe "Documents" do
       fill_in "Full name of the person submitting the proposal", with: "Isabel Garcia"
       documentable_attach_new_file(file_fixture("logo_with_metadata.pdf"))
       check "I agree to the Privacy Policy and the Terms and conditions of use"
-
       click_button "Create proposal"
 
+      expect(page).to have_content "Proposal created successfully"
+
       io = URI.parse("#{app_host}#{polymorphic_path(Document.last.attachment)}").open
       reader = PDF::Reader.new(io)