Bug: Empty sym string for boxplots still displays fliers.

As per documentation, specifying empty string for sym argument should prevent drawing fliers for boxplots.

As an example, the following script displays fliers in the boxplot when it shouldn't.

    import matplotlib.pyplot as plt

    data = list(range(0,100)) + [200,-200,250,-250]
    bp = plt.boxplot(data,sym='',widths=.3)
    plt.show()

Added logic to ensure empty sym string does not display fliers.