fix error of Metadata#date=

[takahashim]

When I add a new spec like below (without fix in lib/gepub/metadata.rb), I got an error:

Failures:

  1) GEPUB::Metadata Generate New OPF should handle date with `date=` method and Time object
     Failure/Error: expect(metadata.date.to_s).to eq('2012-02-27T20:00:00Z')
     
       expected: "2012-02-27T20:00:00Z"
            got: "2012-02-27 20:00:00 UTC"
     
       (compared using ==)
     # ./spec/metadata_spec.rb:160:in `block (3 levels) in <top (required)>'

[coveralls]

Coverage increased (+0.01%) to 95.647% when pulling bc93830 on takahashim:fix-add-date into 56be5a4 on skoji:master.

[coveralls]

Coverage increased (+0.01%) to 95.647% when pulling bc93830 on takahashim:fix-add-date into 56be5a4 on skoji:master.