web: Fix 'send_eof' -> 'write_eof' typo in RuntimeError message

The method has been 'write_eof' since this error message landed in
b462684 (Work on web server, 2014-10-13).
Merge pull request #162 from wking/fix-send-eof-typo

web: Fix 'send_eof' -> 'write_eof' typo in RuntimeError message