StorageProvider spec improvements.

[richardkmichael]

• Re-factor to use implicit subject and review expectations.
• We use mocks with .and_call_original because the methods being
  tested must execute the rest of the method to decorate the returned
  File. Alternately, we could use .and_return(@active_record_file_mock)
  to inject the correct type of file [build it from a Factory, because
  otherwise, the mocked file and the real File will eventually be out of
  sync.
• Remove deep "implementation" testing, e.g. "calls #create!".
• Instead of specing the return behavior of "store!" and
  "retrieve!", should we spec the return behaviour of only "retrieve!"
  and then specifiy that "store!" and "retrieve!" should return the same
  thing?
• DRY URL spec with "shared_examples" / "it_behaves_like" / helper method module?
• Use a named subject to fix the initialization of the StorageProvider under test.
• Remove indirect and deep testing of File via uploader and should_receive (write it as a stub):
  

  carrierwave-activerecord/spec/lib/carrierwave-activerecord/storage/storage_provider_spec.rb

  Line 89 in 0b48c23

  uploader.should_receive(:filename).with(no_args)

carrierwave-activerecord/spec/lib/carrierwave-activerecord/storage/storage_provider_spec.rb

Line 83 in 0b48c23

File.should_receive(:create!).with(file, identifier).and_call_original

carrierwave-activerecord/spec/lib/carrierwave-activerecord/storage/storage_provider_spec.rb

Line 120 in 0b48c23

File.should_receive(:fetch!).with(identifier).and_call_original

• Can we use its with a tag to call a before block?
  

  carrierwave-activerecord/spec/lib/carrierwave-activerecord/storage/storage_provider_spec.rb

  Line 124 in 0b48c23

  # TODO: Can we use "its" with a tag for a before block?