will_paginate should clone query strings

to avoid regenerating a query string over and over, i reuse the same query. for example:

@@my_q = "select custom,other,and,another from cats where blah = 'blah'"

i then do:

Cat.paginate_by_sql(@@my_q, 1, 13)

and deep inside paginate_by_sql (lib/will_paginate/finder.rb:133) it does this:

add_limit! query, :offset => pager.offset, :limit => pager.per_page

which destructively changes the string. the .dup business in the lines above almost avoids modifying the input string, but barely misses.