Exact alignment should allow missing dimension coordinates

Code Sample, a copy-pastable example if possible

import xarray as xr
xr.align(xr.DataArray([1, 2, 3], dims='x'),
         xr.DataArray([1, 2, 3], dims='x', coords=[[0, 1, 2]]),
         join='exact')

Problem description

This currently results in an error, but a missing index of size 3 does not actually conflict:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-15-1d63d3512fb6> in <module>()
      1 xr.align(xr.DataArray([1, 2, 3], dims='x'),
      2          xr.DataArray([1, 2, 3], dims='x', coords=[[0, 1, 2]]),
----> 3          join='exact')

/usr/local/lib/python3.6/dist-packages/xarray/core/alignment.py in align(*objects, **kwargs)
    129                     raise ValueError(
    130                         'indexes along dimension {!r} are not equal'
--> 131                         .format(dim))
    132                 index = joiner(matching_indexes)
    133                 joined_indexes[dim] = index

ValueError: indexes along dimension 'x' are not equal

This surfaced as an issue on StackOverflow: https://stackoverflow.com/questions/51308962/computing-matrix-vector-multiplication-for-each-time-point-in-two-dataarrays

Expected Output

Both output arrays should end up with the x coordinate from the input that has it, like the output of the above expression if join='inner':

(<xarray.DataArray (x: 3)>
 array([1, 2, 3])
 Coordinates:
   * x        (x) int64 0 1 2, <xarray.DataArray (x: 3)>
 array([1, 2, 3])
 Coordinates:
   * x        (x) int64 0 1 2)

Output of xr.show_versions()

Start with the alignment implementation behind xr.align and reproduce the provided join='exact' example. Check how missing dimension indexes are compared, then verify that both arrays receive the existing x coordinate without raising an error.