Make it possible to use multiple-field search operator

Hi! I'm working on an old Rails 2 app which uses Thinking Sphinx 1, and I needed to do a search on either of 2 fields. In other words, I want to find documents which contain a search term in *either* this field *or* that field.

In the Sphinx docs, I found the following:

    multiple-field search operator:
    @(title,body) hello world

This can be accessed in Thinking Sphinx as:

    Model.search :conditions => {'(field1,field2)' => 'value'}

To make this slightly more intuitive, I patched TS so it can also accept:

    Model.search :conditions => {[:field1,:field2] => 'value'}

I don't know if you would like the TS gem to include this functionality. If so, this (small) patch does the trick.