Set field.keys so that delete/undo buttons work

For multiselect fields,
field.key should have a ':' on the end of it - we use as the prefix, not the real key
field.keys should contain the array of real keys being modified

see https://github.com/openstreetmap/iD/blob/master/js/id/ui/preset.js#L17-L45

The address field preset works like this also..
It contains an array of key.fields for all the subfields that might be set.
For multiselect fields, we don't know ahead of time what all the subfields are,
so we append to the list in `change()` and rebuild list in `tags()`

data/presets.yaml

@@ -774,7 +774,7 @@ en:
         # 'railway=*'
         label: Type
       recycling/recycling_choices:
-        # 'recycling=*'
+        # 'recycling:=*'
         label: Accepts
       ref:
         # 'ref=*'

data/presets/fields.json

@@ -1021,7 +1021,7 @@
         "label": "Type"
     },
     "recycling/recycling_choices": {
-        "key": "recycling",
+        "key": "recycling:",
         "type": "multiselect",
         "label": "Accepts"
     },

data/presets/fields/recycling/recycling_choices.json

@@ -1,5 +1,5 @@
 {
-    "key": "recycling",
+    "key": "recycling:",
     "type": "multiselect",
     "label": "Accepts"
 }

js/id/ui/preset/multiselect.js

@@ -9,7 +9,10 @@ iD.ui.preset.multiselect = function(field, context) {
         input,
         isInitialized;
 
-    field.key += ':';
+    // ensure field.key ends with a ':'
+    if (field.key.match(/.*:$/) === null) {
+        field.key += ':';
+    }
 
     function getOptStringKey(val) {
         if (optstrings) {
@@ -35,7 +38,7 @@ iD.ui.preset.multiselect = function(field, context) {
             bObj[obj.key] = obj;
         });
         // Return all elements in a, unless in b
-        return a.filter(function(obj){
+        return a.filter(function(obj) {
             return !(obj.key in bObj);
         });
     }
@@ -71,10 +74,10 @@ iD.ui.preset.multiselect = function(field, context) {
             .each(function() {
                 if (optstrings) {
                     strings = Object.keys(optstrings).map(function(k) {
-                      return {
-                        key: k,
-                        value: field.t('options.' + k, { 'default': optstrings[k] })
-                      };
+                        return {
+                            key: k,
+                            value: field.t('options.' + k, { 'default': optstrings[k] })
+                        };
                     });
                     dispatch.init();
                     isInitialized = true;
@@ -111,12 +114,12 @@ iD.ui.preset.multiselect = function(field, context) {
     function update(data) {
         var chips = multiselectContainer.selectAll('.chips').data(data);
 
-        var chip = chips.enter()
+        var enter = chips.enter()
             .insert('li', 'input')
             .attr('class', 'chips');
 
-        chip.append('span');
-        chip.append('a');
+        enter.append('span');
+        enter.append('a');
 
         chips.select('span').text(function(d) {return d.value;});
 
@@ -130,8 +133,8 @@ iD.ui.preset.multiselect = function(field, context) {
 
     function comboValues(d) {
         return {
-          value: d.value,
-          title: d.value
+            value: d.value,
+            title: d.value
         };
     }
 
@@ -142,6 +145,7 @@ iD.ui.preset.multiselect = function(field, context) {
             var t = {};
             t[field.key + key] = 'yes';
             input.value('');
+            field.keys.push(field.key + key);
             dispatch.change(t);
         }
     }
@@ -175,6 +179,8 @@ iD.ui.preset.multiselect = function(field, context) {
             }
         });
 
+        field.keys = _.map(_.pluck(tagsData, 'key'), function(v) { return field.key + v; });
+
         update(tagsData);
 
         if (isInitialized) {