Fix date-time format. It accepts date times without offset now.

[0xdfgp]

Following the official documentation, the date-time format accepts:

• "1970-01-01T10:05:08"
• "1970-01-01T10:05:08.10"
• "1970-01-01T10:05:08+01:00"

There is an issue with the DateTime format. The regex is actually invalid because the offset is not optional.

This pull request includes:

• Fix to accept date times without offset
• Unitary tests for date-time (there is nothing now)

[0xdfgp]

@sorinsarca thanks for merging the fix. Could you add a new release in order to update our dependencies with composer, please?

[sorinsarca]

@ISD4N version 1.0.12 was just released