Pickle L mode images

[hugovk]

Fixes #1093 with tests.

L mode images couldn't be pickled because they had no palette:

Traceback (most recent call last):
  File "C:\Users\hugovk\github\Pillow\tests\test_pickle.py", line 80, in test_pickle_l_mode
    self.helper_pickle_string(pickle, protocol, mode="L")
  File "C:\Users\hugovk\github\Pillow\tests\test_pickle.py", line 32, in helper_pickle_string
    loaded_im = pickle.loads(dumped_string)
  File "C:\Python27\lib\pickle.py", line 1382, in loads
    return Unpickler(file).load()
  File "C:\Python27\lib\pickle.py", line 858, in load
    dispatch[key](self)
  File "C:\Python27\lib\pickle.py", line 1217, in load_build
    setstate(state)
  File "C:\Python27\lib\site-packages\PIL\Image.py", line 629, in __setstate__
    self.putpalette(palette)
  File "C:\Python27\lib\site-packages\PIL\Image.py", line 1478, in putpalette
    data = "".join(chr(x) for x in data)
TypeError: 'NoneType' object is not iterable

Failing tests:
https://travis-ci.org/hugovk/Pillow/builds/48885366

Passing with fix:
https://travis-ci.org/hugovk/Pillow/builds/48885799