Custom environ variables should not be stringified in wsgihttpexception

[mcdonc]

As reported by plantian:

Exception happened during processing of request from ('127.0.0.1', 54364)
Traceback (most recent call last):
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/paste/httpserver.py", line 1068, in process_request_in_thread
    self.finish_request(request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 323, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python2.7/SocketServer.py", line 639, in __init__
    self.handle()
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/paste/httpserver.py", line 442, in handle
    BaseHTTPRequestHandler.handle(self)
  File "/usr/lib/python2.7/BaseHTTPServer.py", line 337, in handle
    self.handle_one_request()
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/paste/httpserver.py", line 437, in handle_one_request
    self.wsgi_execute()
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/paste/httpserver.py", line 289, in wsgi_execute
    for chunk in result:
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/errormator_client.py", line 658, in __call__
    app_iter = self.app(environ, start_response)
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/pyramid-1.1b1-py2.7.egg/pyramid/router.py", line 187, in __call__
    return response(request.environ, start_response)
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/pyramid-1.1b1-py2.7.egg/pyramid/httpexceptions.py", line 264, in __call__
    self._set_default_attrs(environ)
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/pyramid-1.1b1-py2.7.egg/pyramid/httpexceptions.py", line 239, in _set_default_attrs
    args[k] = escape(v)
  File "build/bdist.linux-x86_64/egg/webob/util.py", line 22, in html_escape
    s = str(s)
  File "/home/ian/workspace/gardentheory/gardentheory/gardentheory/gardentheory/model/user.py", line 78, in __str__
    return self.username
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 168, in __get__
    return self.impl.get(instance_state(instance),dict_)
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 418, in get
    value = callable_(passive)
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/sqlalchemy/orm/state.py", line 275, in __call__
    self.manager.deferred_scalar_loader(self, toload)
  File "/home/ian/workspace/gardentheory/gardentheory/ve/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 2759, in _load_scalar_attributes
    (state_str(state)))
DetachedInstanceError: Instance <User at 0x3637bd0> is not bound to a Session; attribute refresh operation cannot proceed

The key in question here is "gardentheory.user"; we should not try to stringify this (or any other custom environ var) in _set_default_attrs.