datetime: allow boundary values for interval.new

Before this patch, one couldn't create new datetime interval with
boundary value from Lua. At the same time, it was possible to create
such interval from Lua through addition and subtraction. C range
verification allow to create boundary value intervals, error message
also implies that they should be allowed. (See #8878 for more info.)

Closes #8878

NO_DOC=small bug fix

(cherry picked from commit b2a001cc)