Add #set and #[]= methods
Add #set and #[]= methods

#set behaves like xmp_set_property(), allowing the value of a
property to be changed or a new property to be created. Aliased as
set_property for consistency with Exempi and Python XMP Toolkit.

#[]= is sugar on top of #set, allowing a property to be set from a
property name with prefix, e.g. these do the same thing:
xmp.set :tiff, "Make", "Sony"
xmp.set :tiff, "tiff:Make", "Sony"
xmp["tiff:Make"] = "Sony"
mistydemeo/fasttrack