Add small improvements to constrainers and specs

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

lib/constraints/group_url_constrainer.rb

@@ -7,7 +7,7 @@ class GroupUrlConstrainer
     id = extract_resource_path(request.path)
     if id =~ Gitlab::Regex.namespace_regex
-      !!Group.find_by_path(id)
+      Group.find_by(path: id).present?
     else
       false
     end

lib/constraints/user_url_constrainer.rb

@@ -7,7 +7,7 @@ class UserUrlConstrainer
     id = extract_resource_path(request.path)
     if id =~ Gitlab::Regex.namespace_regex
-      !!User.find_by('lower(username) = ?', id.downcase)
+      User.find_by('lower(username) = ?', id.downcase).present?
     else
       false
     end

spec/lib/constraints/group_url_constrainer_spec.rb

@@ -14,6 +14,6 @@ describe GroupUrlConstrainer, lib: true do
   end
   def request(path)
-    OpenStruct.new(path: path)
+    double(:request, path: path)
   end
 end

spec/lib/constraints/user_url_constrainer_spec.rb

@@ -11,6 +11,6 @@ describe UserUrlConstrainer, lib: true do
   end
   def request(path)
-    OpenStruct.new(path: path)
+    double(:request, path: path)
   end
 end