Add test to ensure that XXHash keys are stable.
user
```ruby
# encoding: UTF-8
#
# Copyright (c) 2019 by Chris Schlaeger <chris@taskjuggler.org>
#
# This file contains tests for Array that are similar to the tests for the
# Array implementation in MRI. The ideas of these tests were replicated in
# this code.
#
# MIT License
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
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#
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
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require 'spec_helper'

require 'perobs'

describe PEROBS::XXHash64 do

  before(:all) do
    @xxhash = PEROBS::XXHash64.new(816114511)
  end

  it 'should generate stable hashes for Strings' do
    refs = {
      'foo' => 13888580354979149425,
      'bar' => 11318486070144266075,
      'foobar' => 1136563820614582702,
      'PEROBS rocks your application!' => 4099397047094398144
    }

    refs.each do |s, h|
      expect(@xxhash.digest(s)).to eql(h)
    end
  end

end

```
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