Create append helper function and set $schema as first attribute

[peterdesmet]

Fix #358

Originally written by @PietrH in #362 Edited text from original PR:

I ended up implementing a generic S3 method for append() which allowed me to set a S3 method for append.datapackage(). However, since resource objects don't have a custom class, I had to implement a helper to append lists while retaining attributes. Some attributes can never be kept while appending, such as names (because they depend on the length of the object staying the same, which is not the case for append()). Things like dimnames would also break this custom append helper, as I only added an exception for names.

Furthermore, I re-added tests for the location of the $schema node in upgrade_resource() and upgrade_descriptor(). Both use the same append helper code.

TODO

• Review by @peterdesmet (since he's not the original author)
• Add test (any list)
• Add package test (use check_package()
• Add resource test (expect data_location and path to be retained)
• Review by someone else to pass this
• Rename helper to append_with_attributes()
• See if 036bb52 is reverted. => yes, but separate test to check $schema is first property

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9506648) to head (953bc3c).

Additional details and impacted files

@@            Coverage Diff            @@
##                v2      #366   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines          792       807   +15     
=========================================
+ Hits           792       807   +15     

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[PietrH]

I've added the requested tests, could you have another look @peterdesmet to see if I've covered everything you had in mind?