Fix for Issue #36

The previous formats (```%+.2d%.2d``` and ```%+.2d%C%.2d``` ) assumed the first term will contain the sign of the time-zone offset. 

In practice, the sign (positive or negative) can be present in both terms or in any one of them. 

Hence, the sign should be extracted from the total offset in seconds and, after this, append each term's absolute value to the resulting string.

A unit case for this issue is included as well.