904 | fix CommcareFormBuilder to populate receivedOn field

https://trello.com/c/SyY3ELVd/904-commcare-formbuilder-should-populate-receivedon-field

The receivedOn timestamp comes as part of the request header and motech raises
the form event with it. The CommcareFormBuilder only populates fields from the
actual form. This fix ensures it populates receivedOn field as well.

Change-Id: I89fa5721df54d56dfb8b483f88a96393920804d3