Use the standard name for recursive_open_struct

The gem recursive_open_struct is the standard; recursive-open-struct is
an alias.
Abstract API requests into a private helper method

The method build_request now handles all the logic for building urls and query
parameters, and the default post, put, get, and delete methods now call it.
Update changeloc with helper method and metadata classes
DublinCore and ItemTypeMetadata use singleton methods
Added test to solve problem with class methods in DublinCore
Merge branch 'feature/fix-dc-class-methods' into develop