Wiki macros are rendered in the syntax of the caller document

Use case:

• define a wiki macro in a xwiki/2.1 syntax document, using 2.1 only features, such as image:icon: images
• call the macro in a xwiki/2.0 document
• the icons are not found in the rendered output
• everything works fine if the caller document is also in the xwiki/2.1 syntax