Fix bug and add test for Cancel

The Cancel function was broken in multiple ways. It's fixed now,
with a small test.

Author: kshyatt

src/mpi-base.jl

@@ -456,8 +456,8 @@ function Testsome!(reqs::Array{Request,1})
     (indices, stats)
 end
 
-function Cancel!(res::Request)
-    ccall(MPI_CANCEL, Void, (Ptr{Cint},), &req.val, &0)
+function Cancel!(req::Request)
+    ccall(MPI_CANCEL, Void, (Ptr{Cint},Ptr{Cint}), &req.val, &0)
     req.buffer = nothing
     nothing
 end

test/test_sendrecv.jl

@@ -106,4 +106,10 @@ rreq = nothing
 sreq = nothing
 gc()
 
+recv_mesg = Array(Float64, N)
+sreq = MPI.Isend(send_mesg, dst, rank+32, comm)
+MPI.Cancel!(sreq)
+@test sreq.buffer == nothing
+gc()
+
 MPI.Finalize()