Fix for send_data Content-Length header in Ruby 1.9

In Ruby 1.9 String#size returns the actual character length, not size in bytes. send_data sets Content-Length header to length in characters so when sending multi-byte encoded text the file gets truncated. Example:

send_data("Кирилица", :type => text, :filename => 'utf8_text') # => the browser receives "Кири"

Here is a patch to fix this.