Turns out there are empty strings being passed to the currency filter

So we add handling to the filter where we check the value can be
converted to a valid decimal.  If not, return an empty string.

Fixes #316

oscar/templatetags/currency_filters.py

+from decimal import Decimal as D, InvalidOperation
+
 from django import template
 from django.conf import settings
 from babel.numbers import format_currency
     """
     Format decimal value as currency
     """
-    if value is None:
-        return ''
+    try:
+        value = D(value)
+    except (TypeError, InvalidOperation):
+        return u""
     # Using Babel's currency formatting
     # http://packages.python.org/Babel/api/babel.numbers-module.html#format_currency
     kwargs = {

tests/integration/templatetags/test_currency_filters.py

         }))
         self.assertTrue(u'£10.23' in out)
 
-    def test_handles_missing_values_gracefully(self):
+    def test_handles_none_price_gracefully(self):
         self.template.render(template.Context({
             'price': None
         }))
+
+    def test_handles_string_price_gracefully(self):
+        self.template.render(template.Context({
+            'price': ''
+        }))