[patch] Warning for characters < 0x20 in keys or values

[bilbo]

In the patch I added check for characters with code lower than 0x20 in keys and values to Validator (these are newline, tab and other special characters).

On few occasions I've run into a newline character in the data, causing some trouble in Mapnik and other software, so I thought it would be useful to add check for these and similar cases.

Also, it fixes one bug which could end up with exception (checking for null value should be check for null key)

[bilbo]

Check for characters <0x20 in validator

[stoecker]

In [o21540]. Not yet as binary, due to the previous role changes I did, which probably needs some more testing.