Merge branch 'rails-5-backports' into 'master'

Fix two problematic bits of code that will be deprecated or broken in Rails 5.

Found in the Rails 5 MR: !5555

These are safe to use in Rails 4.2.7 as well as Rails 5.0.0, so I figured I'd backport them for the sake of making that merge request smaller.

The explanation for the mime_types.rb code is here: https://github.com/rails-api/active_model_serializers/issues/1027#issuecomment-126543577

See merge request !6214

app/models/project.rb

@@ -58,7 +58,7 @@ class Project < ActiveRecord::Base
 
   # Relations
   belongs_to :creator, foreign_key: 'creator_id', class_name: 'User'
-  belongs_to :group, -> { where(type: Group) }, foreign_key: 'namespace_id'
+  belongs_to :group, -> { where(type: 'Group') }, foreign_key: 'namespace_id'
   belongs_to :namespace
 
   has_one :last_event, -> {order 'events.created_at DESC'}, class_name: 'Event', foreign_key: 'project_id'

config/initializers/mime_types.rb

@@ -13,9 +13,5 @@ Mime::Type.register "video/mp4",  :mp4, [], [:m4v, :mov]
 Mime::Type.register "video/webm", :webm
 Mime::Type.register "video/ogg",  :ogv
 
-middlewares = Gitlab::Application.config.middleware
-middlewares.swap(ActionDispatch::ParamsParser, ActionDispatch::ParamsParser, {
-  Mime::Type.lookup('application/vnd.git-lfs+json') => lambda do |body|
-    ActiveSupport::JSON.decode(body)
-  end
-})
+Mime::Type.unregister :json
+Mime::Type.register 'application/json', :json, %w(application/vnd.git-lfs+json application/json)