Fix and test case for Issue #946: Edm.DateTime properties are parsed as null if they omit seconds

From the OData specification, (http://www.odata.org/documentation/odata-version-3-0/atom-format) the form of an Edm.DateTime field for both Atom and JSON is:

yyyy “-” mm “-” dd “T” hh “:” mm [":" ss["." fffffff]]

It means that both milliseconds and seconds are optional. However, the Restlet OData client reads correctly only full forms or forms with no milliseconds component. If the seconds are missing, the field is set to null.

This minor pull request fixes the issue and adds a test case for it.