Line 111 fails with the current master branch, reporting the following error:
````
  1) Prawn::Table You can explicitly set the column widths and use a colspan > 1 illustrates issue #407 - comment 28556698
     Failure/Error: table = Prawn::Table.new data, pdf, :column_widths => [50, 200]
     Prawn::Errors::CannotFit:
       Table's width was set larger than its contents' maximum width (max width 250, requested 325.0)
     # ./lib/prawn/table.rb:403:in `column_widths'
     # ./lib/prawn/table.rb:584:in `set_column_widths'
     # ./lib/prawn/table.rb:137:in `initialize'
     # ./spec/table_spec.rb:111:in `new'
     # ./spec/table_spec.rb:111:in `block (3 levels) in <top (required)>'

````
So while technically there is no assert in that line, it does assert, that prawn doesn't raise an error. Is there an assert for that?

Also line 112 asserts (.should) that the calculated column widths are the same as the ones that where requested in line 111.