Pure python BSON decoders don't support memoryviews with offsets

>>> from bson import decode, encode

>>> decode(memoryview(encode({})))

{}

>>> decode(memoryview(b'123'+encode({}))[3:])

Traceback (most recent call last):

  File "/Users/shane/git/mongo-python-driver/bson/__init__.py", line 484, in _bson_to_dict

    _, end = _get_object_size(data, 0, len(data))

  File "/Users/shane/git/mongo-python-driver/bson/__init__.py", line 232, in _get_object_size

    if data[end] != _OBJEND:

IndexError: index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "/Users/shane/git/mongo-python-driver/bson/__init__.py", line 983, in decode

    return _bson_to_dict(data, codec_options)

  File "/Users/shane/git/mongo-python-driver/bson/__init__.py", line 491, in _bson_to_dict

    reraise(InvalidBSON, exc_value, exc_tb)

  File "/Users/shane/git/mongo-python-driver/bson/py3compat.py", line 53, in reraise

    raise exctype(str(value)).with_traceback(trace)

  File "/Users/shane/git/mongo-python-driver/bson/__init__.py", line 484, in _bson_to_dict

    _, end = _get_object_size(data, 0, len(data))

  File "/Users/shane/git/mongo-python-driver/bson/__init__.py", line 232, in _get_object_size

    if data[end] != _OBJEND:

bson.errors.InvalidBSON: index out of range