Call function for emptyTag if specified

In case there are multiple empty tags in xml,
they all share reference to the same default object. This causes unwanted
duplication, when we assign something to any of these empty objects.
When we provide factory function for emptyObject, these objects no longer
share reference and works as expected.

src/parser.coffee

@@ -145,7 +145,7 @@ class exports.Parser extends events.EventEmitter
           obj = obj[charkey]
 
       if (isEmpty obj)
-        obj = if @options.emptyTag != '' then @options.emptyTag else emptyStr
+        obj = @options.emptyTag?() ? (if @options.emptyTag != '' then @options.emptyTag else emptyStr)
 
       if @options.validator?
         xpath = "/" + (node["#name"] for node in stack).concat(nodeName).join("/")

test/fixtures/sample.xml

@@ -55,4 +55,7 @@
     <tagNameProcessTest/>
     <valueProcessTest>some value</valueProcessTest>
     <textordertest>this is text with <b>markup</b>   <em>like this</em> in the middle</textordertest>
+    <emptytestanother>
+
+    </emptytestanother>
 </sample>

test/parser.test.coffee

@@ -72,6 +72,11 @@ module.exports =
     # determine number of items in object
     equ Object.keys(r.sample.tagcasetest[0]).length, 3)
 
+  'test parse with empty objects and functions': skeleton({emptyTag: ()=> new Object()}, (r)->
+    console.log 'Result object: ' + util.inspect r, false, 10
+    bool = r.sample.emptytestanother[0] is r.sample.emptytest[0]
+    equ bool, false)
+
   'test parse with explicitCharkey': skeleton(explicitCharkey: true, (r) ->
     console.log 'Result object: ' + util.inspect r, false, 10
     equ r.sample.chartest[0].$.desc, 'Test for CHARs'