Fixes issue in devserver introduced in v3.3

Fixes an intermittent devserver problem with directory urls
containing index.html (i.e. clean urls).  It tries to send the
index.html file twice, resulting in a scrambled web page complete
with HTTP headers in the output, and sometimes Broken Pipe errors.
Merge pull request #1178 from robulouski/devserver_fix

Fix issue in devserver introduced in v3.3