decode dict keys only if strings

[ltalirz]

fix #1720
backport of #2017

[coveralls]

Coverage increased (+0.001%) to 55.802% when pulling e3bf12c on ltalirz:issue_1720_serialize_dict_integer_keys into dde378e on aiidateam:release_v0.12.3.

[ltalirz]

@yakutovicha as discussed I won't be adding a test (serialize functions are incorrectly in aiida.utils although they use the DB; adding a test to verdi devel tests aiida.utils did not work and I don't see the point in messing around with this)

In lieu of an actual test, here the shell commands

In [3]: d={ 1:'123213'}

In [4]: from aiida.utils.serialize import serialize_data

In [5]: serialize_data(d)
Out[5]: {1: '123213'}

[yakutovicha]

it is ok