* Avoid some reference cycles through tracebacks in httpserver.py

* Handle io.UnsupportedOperation from socket.tell()

On Python 3, socket.makefile() returns an object with a tell() method, but one that always raises io.UnsupportedOperation.

If we don't want to parse the request body with cgi, we can just not use cgi rather than building a fake request.

* Ensure unicode URLs work in TestApp.
* Make LimitedLengthFile file return empty bytes.
* Protect against accidental close in FieldStorage.

Passing a unicode url to get() always worked (assuming the url contained only ASCII), but it didn't work to post(), put(), or delete() if query parameters were included. This change fixes the latter cases.

On Python 3, cgi.FieldStorage has a __del__ method that closes the underlying file [1]. This means that if the copy made from UnicodeMultiDict._decode_value is garbage collected, the file underlying the original FieldStorage will be closed! Fix this by not copying FieldStorage if it is not required by decode_keys=False. I cannot think of a nice way to fix this problem if decode_keys=True.

[1] https://github.com/python/cpython/commit/f79126f373a9d5c9b584a8db736fe490fcbfa77a