Corrected EntityFunctions.DiffMinutes support.

The implementation of sql generation for EntityFunctions.DiffXXXXX was incorrect.

In my case using of EntityFunctions.DiffMinutes with EF 4.3 generate script code
{code}
("time"-((E'2013-01-01 00:00:00.000000')) * INTERVAL '1 Minutes')
{code}
that causes postgres error
{code}
ERROR:  invalid syntax for type double precision: "2013-01-01 00:00:00.000000"
LINE 1: select ("time"-((E'2013-01-01 00:00:00.000000')) * INTERVAL ...
{code}
Unfortunately I don't have time for implementing unit tests but EntityFunctions.DiffMinutes works correct in my project.