Allow abstract parameters in Intervals again

When combined with SIUnits, whose inference is poor, Intervals break. I think disallowing this from the constructor is sufficient in most cases.

Author: mbauman

src/intervals.jl

@@ -36,7 +36,6 @@ immutable Interval{T}
     lo::T
     hi::T
     function Interval(lo, hi)
-        isleaftype(T) || throw(ArgumentError("cannot construct Interval with an abstract type parameter"))
         lo <= hi ? new(lo, hi) : throw(ArgumentError("lo must be less than or equal to hi"))
     end
 end

test/intervals.jl

@@ -9,7 +9,6 @@
 v = [1 .. 2, 3.0 .. 4.0]
 @test v[1] === 1.0 .. 2.0
 @test v[2] === 3.0 .. 4.0
-@test_throws ArgumentError [1 .. 2, Dates.today() .. Dates.today()]
 
 # Test simple arithmetic, with promotion behaviors
 @test (1.0 .. 2.0) + 1 === (2.0 .. 3.0)