FIX: issue 99

[henrymartin1]

Fixes #99

The bins keyword is popped in plot_pdf and given explicitly to the pdf function. As there is only 1 keyword available in the pdf function I thought that this would be the easiest solution. Otherwise I would not be sure how to pop the bins keyword globally from within the function. I have created a test that checks if the bins keyword works as expected but I was not sure about the naming conventions etc. So feel free to comment or edit!