resolves #214 use table background color, even if white
resolves #214 use table background color, even if white

- use the table background color, even if FFFFFF (i.e, white)
- remove logic to force table background to FFFFFF when nested inside a block
- if both the table bg color and one of the row colors is nil, then
  color from the alternate row will bleed through on rowspans (prawn-table issue 45)

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