Return page contents as list not string

Returning content as a string causes WSGIref to iterate over it character by character and calling sendto() for each character.This takes forever for large responses like 20K transactions in an address lookup.

This does not seem to affect things returned by serve_static()

Returning as a list fixes that. sendto() calls are each 8192 bytes long as seen with strace
![Uploading abe_sendto1char.PNG . . .]()
![Uploading abe_sendto8192char.PNG . . .]()