Imrpove test coverage + use zero()

Author: petvana

src/intervals/functions.jl

@@ -378,7 +378,7 @@ end
 Calculate `x mod y` where `x` is an interval and `y` is a positive divisor.
 """
 function mod(x::Interval, y::Real)
-    @assert y > 0 "modulo is currently implemented only for a positive divisor."
+    @assert y > zero(y) "modulo is currently implemented only for a positive divisor."
     division = x / y
     fl = floor(division)
     fl.lo < fl.hi ? Interval(zero(y), y) : y * (division - fl)

test/interval_tests/numeric.jl

@@ -458,4 +458,6 @@ end
     @test mod(x, 2) == mod(x, 2.0) == 0..2
     @test mod(x, 2.5) == 0..2.5
     @test mod(x, 0.5) == 0..0.5
+
+    @test_throws AssertionError mod(x, -1)
 end