Draw top border of Table box when show_header=false

[GregPlowman]

Issue #242

Change so that top border of Table box is drawn when show_header=false

julia> data = reshape(1:12, 3, 4)
3×4 reshape(::UnitRange{Int64}, 3, 4) with eltype Int64:
 1  4  7  10
 2  5  8  11
 3  6  9  12

Before:

julia> Table(data; box=:ROUNDED, show_header=false)
│  1  │  4  │  7  │  10  │
├─────┼─────┼─────┼──────┤
│  2  │  5  │  8  │  11  │
├─────┼─────┼─────┼──────┤
│  3  │  6  │  9  │  12  │
╰─────┴─────┴─────┴──────╯

After:

julia> Table(data; box=:ROUNDED, show_header=false)
╭─────┬─────┬─────┬──────╮
│  1  │  4  │  7  │  10  │
├─────┼─────┼─────┼──────┤
│  2  │  5  │  8  │  11  │
├─────┼─────┼─────┼──────┤
│  3  │  6  │  9  │  12  │
╰─────┴─────┴─────┴──────╯

[t-bltg]

This PR needs some tests added in the test suite.