Relation.replaceMember shouldn't necessarily remove duplicates

[bhousel]

Also related to #4589

Our current replaceMember behavior removes duplicates from the relation list - I think this is wrong in the context of a Public Transport route relation. We call this function numerous places in the iD code when working with relations.

iD/test/spec/osm/relation.js

Lines 274 to 279 in fdf1fa4

	it('removes members if replacing them would produce duplicates', function () {
	var r = iD.Relation({members: [
	{id: 'a', role: 'b', type: 'node'},
	{id: 'b', role: 'b', type: 'node'}]});
	expect(r.replaceMember({id: 'a'}, {id: 'b', type: 'node'}).members).to.eql([{id: 'b', role: 'b', type: 'node'}]);
	});

Here is an example of a route that might break if we call replaceMember on it:

Route: A - B - C - B - D

// 'B' is duplicate but should not be removed!
r = iD.osmRelation({members: [
    {id: 'A', role: 'forward', type: 'way'},
    {id: 'B', role: 'forward', type: 'way'},
    {id: 'C', role: 'forward', type: 'way'},
    {id: 'B', role: 'forward', type: 'way'},
    {id: 'D', role: 'forward', type: 'way'}
]});

more reading:

• https://wiki.openstreetmap.org/wiki/Public_transport#Service_routes
• https://wiki.openstreetmap.org/wiki/Proposed_features/Simplified_Public_Transport_Scheme#Spoon_lines