Added getIndexOfQueryTail function. Returns the index that is likely …

[bjornmosten]

…to be right after the file extension in the given URI. The function only allows filename to be extracted from the last forward slash to a query divider if the query divider appears after the forward slash in the given string (URI).

This makes URI:s like https://example.com/img?path=/photos/example&w=1000 work with this package, whereas previously the filename would become something like path=/photos which appears to reference a non-existent directory, breaking caching.