Convert sendfile param to string from bool.

Currently, one would assume that since the default is false, setting `sendfile
=> true` would cause the option to be enabled in httpd.conf, but instead it
writes `sendfile true` which is incorrect.  This makes the usage consistant
with commit a82af5595e1e51961903873b6171bd5e08857d60.  Includes rspec
tests and input validation.