Don't manually set Content-Length in StaticFileHandler.

Content-Length will be set automatically in RequestHandler.finish.  This
change fixes a bug in which RequestHandler.finish's Etag support strips
the response body without changing any headers.  Transfer headers
(including Content-Length and Transfer-Encoding) are the responsibility
of the framework, not the "application-level" handlers (which includes
StaticFileHandler).

Closes #160.

tornado/web.py

@@ -1284,7 +1284,6 @@ class StaticFileHandler(RequestHandler):
 
         if not include_body:
             return
-        self.set_header("Content-Length", stat_result[stat.ST_SIZE])
         file = open(abspath, "rb")
         try:
             self.write(file.read())