ExampleTable::Header used to calculate column width.

This fixes a bug in the pretty formatter.

If you have a ExamplesTable where the header row cells where longer than example
row cells the formatter would not be able to calculate the correct padding for
the table cells. This would cause an error like

    negative argument (ArgumentError)
    /Users/tom025/ruby/cucumber/lib/cucumber/formatter/pretty.rb:202:in `*'
    /Users/tom025/ruby/cucumber/lib/cucumber/formatter/pretty.rb:202:in `table_cell_value'