Problem with indexers in plot operations

[forman]

All plot_ operations have an indexer input allowing callers to specify the data slice to be plotted given dim1=label1, dim2=label2, ... of a coordinate variable dim.

Expected behavior

If indexers is given, xarray indexing methods should always be nearest.

Actual behavior

Indexing methods is nearest only if a label is not of type int. If user types lat=12 the plot call will fail, because ``12is anint`.

Steps to reproduce the problem

See unit tests in plotting.

Specifications

0.9.0dev3