Workaround bug in lxml for multiple select options

In lxml version pre 2.3.1 there is a bug that returns
all options elements instead of those selected for select tag

it is mentioned in 2.3.1 release notes
http://lxml.de/2.3/changes-2.3.1.html

and fixed by
57f49eed82 (L1R1139)

scrapy/http/request/form.py

@@ -93,16 +93,25 @@ def _get_inputs(form, formdata, dont_click, clickdata, response):
 def _value(ele):
     n = ele.name
     v = ele.value
-    # Match browser behaviour on simple select tag without options selected
-    # Or for select tags wihout options
-    if v is None and ele.tag == 'select' and not ele.multiple:
-        o = ele.value_options
-        if o:
-            return n, o[0]
-        else:
-            return None, None
+    if ele.tag == 'select':
+        return _select_value(ele, n, v)
     return n, v
 
+def _select_value(ele, n, v):
+    multiple = ele.multiple
+    if v is None and not multiple:
+        # Match browser behaviour on simple select tag without options selected
+        # And for select tags wihout options
+        o = ele.value_options
+        return (n, o[0]) if o else (None, None)
+    elif v is not None and multiple:
+        # This is a workround to bug in lxml fixed 2.3.1
+        # fix https://github.com/lxml/lxml/commit/57f49eed82068a20da3db8f1b18ae00c1bab8b12#L1L1139
+        selected_options = ele.xpath('.//option[@selected]')
+        v = [(o.get('value') or o.text or u'').strip() for o in selected_options]
+    return n, v
+
+
 def _get_clickable(clickdata, form):
     """
     Returns the clickable element specified in clickdata,