Cannot acquire a node AttributeError: This QueryDict instance is immutable

[Blake Rouse]

This is currently breaking trunk and causing Juju to fail to acquire a node in CI.

2015-08-07 08:48:10 [maasserver] ERROR: ################################ Exception: This QueryDict instance is immutable ################################
2015-08-07 08:48:10 [maasserver] ERROR: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/support.py", line 53, in __call__
    response = upcall(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/decorators/vary.py", line 19, in inner_func
    response = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/piston/resource.py", line 167, in __call__
    result = self.error_handler(e, request, meth, em_format)
  File "/usr/lib/python2.7/dist-packages/piston/resource.py", line 165, in __call__
    result = meth(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/support.py", line 201, in dispatch
    return function(self, request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/nodes.py", line 1285, in acquire
    request, extract_params=True)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/nodes.py", line 208, in get_storage_layout_params
    storage_layout = request.data.pop('storage_layout', None)
  File "/usr/lib/python2.7/dist-packages/django/http/request.py", line 364, in pop
    self._assert_mutable()
  File "/usr/lib/python2.7/dist-packages/django/http/request.py", line 322, in _assert_mutable
    raise AttributeError("This QueryDict instance is immutable")
AttributeError: This QueryDict instance is immutable

2015-08-07 08:48:11 [-] "127.0.0.1" - - [07/Aug/2015:12:48:10 +0000] "POST /MAAS/api/1.0/nodes/?op=acquire HTTP/1.1" 500 36 "-" "Go 1.1 package http"
2015-08-07 08:48:11 [maasserver] ERROR: ################################ Exception: This QueryDict instance is immutable ################################
2015-08-07 08:48:11 [maasserver] ERROR: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/support.py", line 53, in __call__
    response = upcall(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/decorators/vary.py", line 19, in inner_func
    response = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/piston/resource.py", line 167, in __call__
    result = self.error_handler(e, request, meth, em_format)
  File "/usr/lib/python2.7/dist-packages/piston/resource.py", line 165, in __call__
    result = meth(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/support.py", line 201, in dispatch
    return function(self, request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/nodes.py", line 1285, in acquire
    request, extract_params=True)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/nodes.py", line 208, in get_storage_layout_params
    storage_layout = request.data.pop('storage_layout', None)
  File "/usr/lib/python2.7/dist-packages/django/http/request.py", line 364, in pop
    self._assert_mutable()
  File "/usr/lib/python2.7/dist-packages/django/http/request.py", line 322, in _assert_mutable
    raise AttributeError("This QueryDict instance is immutable")
AttributeError: This QueryDict instance is immutable