fill value not appearing in show

It would be useful to display the fill value when printing a GBMatrix
```julia
julia> x = GBMatrix([1,2], [2, 3], [1,2], fill=17)
2x3 GraphBLAS int64_t matrix, bitmap by row
  2 entries, memory: 264 bytes

    (1,2)   1
    (2,3)   2
```