BUG: type change breaks BlockManager integrity

closes https://github.com/pydata/pandas/issues/8850

on master:

    >>> cols = MultiIndex.from_tuples([('1st', 'a'), ('2nd', 'b'), ('3rd', 'c')])
    >>> df = DataFrame([[1.0, 2, 3], [4.0, 5, 6]], columns=cols)
    >>> df['2nd'] = df['2nd'] * 2.0  # type change in block manager
    /usr/lib/python3.4/site-packages/numpy/lib/function_base.py:3612: FutureWarning: in the future negative indices will not be ignored by `numpy.delete`.
      "`numpy.delete`.", FutureWarning)
    >>> df.values
    ...
      File "/usr/lib/python3.4/site-packages/pandas-0.15.1_72_gf504885-py3.4-linux-x86_64.egg/pandas/core/internals.py", line 2392, in _verify_integrity
        tot_items))
    AssertionError: Number of manager items must equal union of block items
    # manager items: 3, # tot_items: 4
    >>> df.blocks
    ...
      File "/usr/lib/python3.4/site-packages/pandas-0.15.1_72_gf504885-py3.4-linux-x86_64.egg/pandas/core/internals.py", line 2392, in _verify_integrity
        tot_items))
    AssertionError: Number of manager items must equal union of block items
    # manager items: 3, # tot_items: 4
`._data` is also broken:

    >>> df._data
    BlockManager
    Items:       
    1st  a
    2nd  b
    3rd  c
    Axis 1: Int64Index([0, 1], dtype='int64')
    FloatBlock: slice(0, 1, 1), 1 x 2, dtype: float64
    IntBlock: slice(1, 3, 1), 2 x 2, dtype: int64
    FloatBlock: slice(1, 2, 1), 1 x 2, dtype: float64