Refactor trace api_update to avoid duplicating tags

[gravitystorm]

This PR refactors the trace api_update method to work on existing trace objects, rather than creating temporary 'shadow' trace objects. In doing so it refactors the from_xml and from_xml_node model methods to be instance methods instead of class methods.

Advantages:

• No longer duplicate tags (i.e. fixes PUT on gpx_file in API does not remove old tags #1600)
• makes updating a trace via xml more similar to updating via forms (which don't use shadow objects)

Concerns:

• Doesn't avoid deleting and recreating tag objects when the tags are the same (but neither does the tagstring= method, afaict)
• Other classes (Node, Changeset etc) all use the class method approach for from_xml, rather than instance methods. I don't know whether they should be refactored too, or whether there's some advantage of the class method approach that I haven't understood.

[tomhughes]

Assigning to tags is enough to trigger deletion of the old tags I take it?

There is of course a fundamental difference between traces and other objects in that we don't keep multiple versions of traces - so it makes sense to have from_xml on a node be a constructor that creates the new object we are going to save that replaces the current version. Though I guess we don't keep history for changesets either.

I certainly think that if we go this route we should change the method name to update_from_xml or something so that we don't have different models using the same name in different ways.

[gravitystorm]

Assigning to tags is enough to trigger deletion of the old tags I take it?

Yes. From http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many :

"collection=objects : Replaces the collections content by deleting and adding objects as appropriate."

I certainly think that if we go this route we should change the method name to update_from_xml or something so that we don't have different models using the same name in different ways.

OK, I can do that.

[tomhughes]

Looks good to me now.