Error when saving session with custom helper function

[astrofrog]

I tried saving a session that makes use of a custom @link_helper function from config.py and got the following error:

Traceback (most recent call last):
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/application_base.py", line 25, in wrapper
    return func(*args, **kwargs)
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/application_base.py", line 94, in save_session
    state = gs.dumps(indent=2)
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 338, in dumps
    result = self.dumpo()
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 320, in dumpo
    return self.do_all()
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 254, in do_all
    result = [(oid, self.do(obj)) for oid, obj in list(self._objs.items())]
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 254, in <listcomp>
    result = [(oid, self.do(obj)) for oid, obj in list(self._objs.items())]
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 276, in do
    result = fun(obj, self)
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 715, in _save_component_link
    using = context.do(link.get_using())
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 276, in do
    result = fun(obj, self)
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/link_helpers.py", line 58, in __gluestate__
    return dict(func=context.do(self.func), index=self.index)
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 274, in do
    fun, version = self._dispatch(obj)
  File "/Users/tom/miniconda3/envs/production/lib/python3.4/site-packages/glueviz-0.6.0.dev2176-py3.4.egg/glue/core/state.py", line 301, in _dispatch
    " %r of type %s" % (obj, type(obj)))
glue.core.state.GlueSerializeError: Don't know how to serialize <bound method GalactocentricToGalactic.backward of [l <- GalactocentricToGalactic.forward_1(x, y, z), b <- GalactocentricToGalactic.forward_2(x, y, z), d <- GalactocentricToGalactic.forward_3(x, y, z), x <- GalactocentricToGalactic.backward_1(l, b, d), y <- GalactocentricToGalactic.backward_2(l, b, d), z <- GalactocentricToGalactic.backward_3(l, b, d)]> of type <class 'method'>

[astrofrog]

Fixed by #1139