Don't mutate the options hash

[rap1ds]

Problem: #enqueue method mutates the options hash which includes priority, queue name, etc. This causes problems, if the same hash is used to create another job. In general, mutating method arguments is unexpected and a bad practise (might be a matter of taste but at least in my opinion it is a bit bad).

Because of this, creating a second job with the same options fails and throws an error ActiveRecord::StatementInvalid: SQLite3::ConstraintException: NOT NULL constraint failed

In my application when I first discovered this issue, my DB schema was a bit outdated and it allowed NULL values for priority. The job was successfully written to the database, but it contained the parameters that belonged to the first job (that is, id: 1 instead of id: 2, see the steps to reproduce).

Solution: Clone the hash before mutating it

Steps to reproduce:

> rails --version
# Rails 4.2.5.1
> rails new dont-mutate
> cd dont-mutate
> echo "gem 'delayed_job', :git => 'https://github.com/collectiveidea/delayed_job.git'" >> Gemfile
> echo "gem 'delayed_job_active_record'" >> Gemfile
> bundle install
> rails generate delayed_job:active_record
> rake db:migrate
> mkdir app/utils/
> touch app/utils/runner.rb

# app/utils/runner.rb
class Runner
  Job = Struct.new(:id) do |id|
    def perform
      # do something with the id
    end
  end

  DEFAULT_OPTIONS = {
    priority: 1,
    queue: "important"
  }

  def run(id:)
    Delayed::Job.enqueue(Job.new(id), DEFAULT_OPTIONS)
  end
end

> rails console
console> require_relative './app/utils/runner'
console> @runner = Runner.new
console> @runner.run(id: 1)

# Expected: New job created with id: 1, priority: 1, queue: "important"
# Actual: New job created with id: 1, priority: 1, queue: "important"

# Try again:
console> @runner.run(id: 2)

# Expected: New job created with id: 2, priority: 1, queue: "important"
# Actual:

[DEPRECATION] Passing multiple arguments to `#enqueue` is deprecated. Pass a hash with :priority and :run_at.
   (0.1ms)  begin transaction
  SQL (1.2ms)  INSERT INTO "delayed_jobs" ("priority", "queue", "handler", "run_at", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?, ?)  [["priority", nil], ["queue", "important"], ["handler", "--- !ruby/struct:Runner::Job\nid: 1\n"], ["run_at", "2016-02-06 11:11:00.593972"], ["created_at", "2016-02-06 11:11:00.594317"], ["updated_at", "2016-02-06 11:11:00.594317"]]
   (0.1ms)  rollback transaction
ActiveRecord::StatementInvalid: SQLite3::ConstraintException: NOT NULL constraint failed: delayed_jobs.priority: INSERT INTO "delayed_jobs" ("priority", "queue", "handler", "run_at", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?, ?)

[rap1ds]

Thanks for merging! Sorry about the Rubocop errors.