restore previous behaviour with \n in strings

An incompatible change (that actually breaks po parsing after writing) had
been introduced with commit e164fcfe9d6f28cd3d452f4de274a41663160cef. If
_process returned an array (which is the case for strings containing \n
character), array.toString will return a comma separated list, which is not
valid po syntax. Added a test to restore the behaviour from before the
e164fcfe9d6f28cd3d452f4de274a41663160cef.