If an array with same numbers is given as plot data, it fails

[KarlHeitmann]

A float domain error appears if you pass an array with same numbers. Eg: AsciiChart([4,4,4,4]). You get this:

AsciiChartTest#test_writes_array_same_numbers:
FloatDomainError: NaN
   ascii_chart/lib/ascii_chart/line.rb:25:in `ceil'
   ascii_chart/lib/ascii_chart/line.rb:25:in `plot'
   ascii_chart/lib/ascii_chart.rb:8:in `plot'
   ascii_chart/test/ascii_chart_test.rb:12:in `test_writes_array_same_numbers'

If you pass height params, you get a similar error, but instead of FloatDomainError: NaN, you get INFINITY problem. I'm going to upload a PR to solve this issue...