Indicate that certain XML element order is unimportant in comparison

[ronaldtse]

From:

• Round trip all records in spec/fixtures relaton/loc_mods#7

There are a number of files that are currently skipped due to the serialization order being wrong, however, changing the order of those elements will cause more failures since the order in the test dataset is inconsistent.

I also don't know how to ignore element order comparison when it happens to be insignificant.

https://github.com/relaton/loc_mods/blob/110e14ea92eb4509bcd3badf01a78a5564c735b1/spec/loc_mods/collection_spec.rb#L12-L62

[ronaldtse]

There are a few types of ordering in XML:

• sequence: an element <X> must follow <Y>
• orderless sequence: <X> and <Y> can show up in any sequence.
• collection: the same <X> element if there are multiple, they are ordered.