title: Prawn::Errors::CannotFit: Table's width was set too small to contain its contents
body: <p>I am trying to create a table in Prawn using the following code however the columns widths do not suite well for me with columns width I have now.  Given that I still have some free space, I want to expand a little bit the second and last columns so I increased both of them to <code>55.mm</code> and <code>20.mm</code> accordingly but I am getting an error</p>
  
  <p><code>Prawn::Errors::CannotFit: Table's width was set too small to contain its contents (min width 450.70866141732284, requested 442)</code>. </p>
  
  <p>Why does expanding a column width throws <code>Tabe too small</code> error?</p>
  
  <p>I have calculated the total width of the columns:</p>
  
  <pre><code>9.mm + 45.mm + 20.mm + 20.mm + 20.mm + 30.mm + 15.mm + 20.mm #=&gt; 507.40157480314963
  </code></pre>
  
  <p>I don't understand it. What am I doing wrong?</p>
  
  <pre><code>bounding_box [0, 792], width: 612, height: 792 do
    bounding_box [30, 792], width: 442, height: 792 do
      line_items_table
    end
  end
  
  def line_items_table
    table line_item_rows, table_options do
      row(0).background_color = TABLE_BACKGROUND_COLOR
      columns(2..-1).align = :right
    end
  end
  
  def table_options
    {
      cell_style: { border_width: 0 },
      column_widths: TABLE_COLUMN_WIDTHS,
      header: true,
      # width: 160.mm
    }
  end
  
  def line_item_rows
    [TABLE_HEADERS]
  end
  
  TABLE_COLUMN_WIDTHS = { 0 =&gt; 9.mm, 1 =&gt; 45.mm, 2 =&gt; 20.mm, 3 =&gt; 20.mm, 4 =&gt; 30.mm, 5 =&gt; 15.mm, 6 =&gt; 15.mm }.freeze
  
  TABLE_HEADERS  = [I18n.t('pdf.headers.row_num'), I18n.t('pdf.headers.name'),
                         I18n.t('pdf.headers.quantity'), I18n.t('pdf.headers.price_net_amount'), I18n.t('pdf.headers.net_amount'),
                         I18n.t('pdf.headers.vat'), I18n.t('pdf.headers.gross_amount')].freeze
  #Table headers above are: 'No', 'Name', 'Qauntity', 'Net Sale', 'Net amount', 'Gross amount', 'VAT'
  </code></pre>