title: Flask: 'Response' object is not iterable with response-producing exceptions
body: <p>I can't seem to generate responses from exceptions anymore in Flask 0.10.1 (the same happened with 0.9). This code:</p>
  
  <pre><code>from flask import Flask, jsonify
  from werkzeug.exceptions import HTTPException
  import flask, werkzeug
  
  print 'Flask version: %s' % flask.__version__
  print 'Werkzeug version: %s' % werkzeug.__version__
  
  app = Flask(__name__)
  app.config['PROPAGATE_EXCEPTIONS'] = True
  
  class JSONException(HTTPException):
      response = None
  
      def get_body(self, environ):
          return jsonify(a=1)
  
      def get_headers(self, environ):
          return [('Content-Type', 'application/json')]
  
  @app.route('/x')
  def x():
      return jsonify(a=1)
  
  @app.route('/y')
  def y():
      raise JSONException()
  
  c = app.test_client()
  r = c.get('x')
  print r.data
  r = c.get('y')
  print r.data
  </code></pre>
  
  <p>prints</p>
  
  <pre><code>Flask version: 0.10.1
  Werkzeug version: 0.9.4
  {
    "a": 1
  }
  Traceback (most recent call last):
    File "flask_error.py", line 33, in &lt;module&gt;
      print r.data
    File "/home/path/lib/python2.7/site-packages/werkzeug/wrappers.py", line 881, in get_data
      self._ensure_sequence()
    File "/home/path/lib/python2.7/site-packages/werkzeug/wrappers.py", line 938, in _ensure_sequence
      self.make_sequence()
    File "/home/path/lib/python2.7/site-packages/werkzeug/wrappers.py", line 953, in make_sequence
      self.response = list(self.iter_encoded())
    File "/home/path/lib/python2.7/site-packages/werkzeug/wrappers.py", line 81, in _iter_encoded
      for item in iterable:
    File "/home/path/lib/python2.7/site-packages/werkzeug/wsgi.py", line 682, in __next__
      return self._next()
    File "/home/path/lib/python2.7/site-packages/werkzeug/wrappers.py", line 81, in _iter_encoded
      for item in iterable:
    File "/home/path/lib/python2.7/site-packages/werkzeug/wsgi.py", line 682, in __next__
      return self._next()
    File "/home/path/lib/python2.7/site-packages/werkzeug/wrappers.py", line 81, in _iter_encoded
      for item in iterable:
  TypeError: 'Response' object is not iterable
  </code></pre>
  
  <p>The traceback is unexpected.</p>