Requesting SVGs from a plugins resources directory doesn't work
Actually, the resourceUrl() method doesn't care about the resource file's extension and is able to serve up SVGs (and any other filetype) just fine, so there must be something else wrong. Most likely, the path (i.e. cocktailrecipes/images/gin.svg) to the file isn't correct. 

If your plugin has the handle cocktailrecipes (all lowercase; the handle should be identical to your plugin's folder under /craft/plugins), for your particular example Craft will expect to find your file under /craft/plugins/cocktailrecipes/resources/images/gin.svg – first thing to check, is if that file is actually in that location.