Fix mid using specialized version for alpha=0.5 (#24)

* Fix IntervalBox subtyping; comment out intervalbox_macro

* Try using a VERSION test to define IntervalBox with correct subtyping on 0.5 and 0.6

* Specialised version of mid for alpha=0.5

src/intervals/arithmetic.jl

@@ -312,7 +312,7 @@ doc"""
 
 Find the midpoint (or, in general, an intermediate point) at a distance α along the interval `a`. The default is the true midpoint at α=0.5.
 """
-function mid{T}(a::Interval{T}, α=0.5)
+function mid{T}(a::Interval{T}, α)
 
     isempty(a) && return convert(T, NaN)
     isentire(a) && return zero(a.lo)
@@ -325,6 +325,19 @@ function mid{T}(a::Interval{T}, α=0.5)
     return (1-α) * a.lo + α * a.hi  # rounds to nearest
 end
 
+
+function mid{T}(a::Interval{T})  # specialized version for α=0.5
+
+    isempty(a) && return convert(T, NaN)
+    isentire(a) && return zero(a.lo)
+
+    a.lo == -∞ && return nextfloat(a.lo)
+    a.hi == +∞ && return prevfloat(a.hi)
+
+    return 0.5 * (a.lo + a.hi)  # rounds to nearest
+end
+
+
 mid{T}(a::Interval{Rational{T}}) = (1//2) * (a.lo + a.hi)