Fix specs to RSpec 3.1.7 syntax with Transpec

Fix Rspec syntax after merging https://github.com/jnunemaker/httparty/pull/313
This conversion is done by Transpec 3.0.3 with the following command:
    transpec

* 4 conversions
    from: obj.should
      to: expect(obj).to

For more details: https://github.com/yujinakayama/transpec#supported-conversions