Allow an optional options object to be passed to stringify. One option is now supported, allowDuplicatePrefixes: true/false. Default behaviour remains as before, where array indicies are appended to serialised output (e.g. "a[0]=1&a[1]=2"). Specifying true prevents array indicies being added, allowing duplicate name=value pairs in the query string, e.g. "a=1&a=2".