BUG: preserve left frame order in left merge

closes https://github.com/pydata/pandas/issues/7331

on master:

    >>> left
          dates states
    0  20140101     CA
    1  20140102     NY
    2  20140103     CA
    >>> right
       stateid states
    0        1     CA
    1        2     NY
    >>> pd.merge(left, right, how='left', on='states', sort=False)
          dates states  stateid
    0  20140101     CA        1
    1  20140103     CA        1
    2  20140102     NY        2
`DataFrame.join` already works fine:

    >>> left.join(right.set_index('states'), on='states', how='left')
          dates states  stateid
    0  20140101     CA        1
    1  20140102     NY        2
    2  20140103     CA        1

on branch:

    >>> pd.merge(left, right, how='left', on='states', sort=False)
          dates states  stateid
    0  20140101     CA        1
    1  20140102     NY        2
    2  20140103     CA        1

    >>> pd.merge(left, right, how='left', on='states', sort=True)
          dates states  stateid
    0  20140101     CA        1
    1  20140103     CA        1
    2  20140102     NY        2