don't promote Integer^BigInt to BigInt

Fixes the following inconsistency:

    julia> [0x2^T(9) for T in (Int16,Int32,Int64,BigInt)]
    4-element Array{Any,1}:
     0x00
     0x00
     0x00
      512

base/gmp.jl

@@ -414,7 +414,6 @@ end
 ^(x::BigInt , y::BigInt ) = bigint_pow(x, y)
 ^(x::BigInt , y::Bool   ) = y ? x : one(x)
 ^(x::BigInt , y::Integer) = bigint_pow(x, y)
-^(x::Integer, y::BigInt ) = bigint_pow(BigInt(x), y)
 
 function powermod(x::BigInt, p::BigInt, m::BigInt)
     p < 0 && throw(DomainError())

test/numbers.jl

@@ -2509,3 +2509,5 @@ for (d,B) in ((4//2+1im,Rational{BigInt}),(3.0+1im,BigFloat),(2+1im,BigInt))
     @test typeof(big([d])) == Vector{Complex{B}}
     @test big([d]) == [d]
 end
+
+@test 0x2^9 === 0x2^big(9) === 0x0