Make to_xpath stringize a non-string operand as the standard filters do.

app/concerns/liquid_interpolatable.rb

@@ -52,7 +52,7 @@ module LiquidInterpolatable
 
     # Escape a string for use in XPath expression
     def to_xpath(string)
-      subs = string.scan(/\G(?:\A\z|[^"]+|[^']+)/).map { |x|
+      subs = string.to_s.scan(/\G(?:\A\z|[^"]+|[^']+)/).map { |x|
         case x
         when /"/
           %Q{'#{x}'}

spec/concerns/liquid_interpolatable_spec.rb

@@ -53,5 +53,10 @@ describe LiquidInterpolatable::Filters do
         @filter.to_xpath_roundtrip(string).should == string
       }
     end
+
+    it 'should stringize a non-string operand' do
+      @filter.to_xpath_roundtrip(nil).should == ''
+      @filter.to_xpath_roundtrip(1).should == '1'
+    end
   end
 end