keyword "where" added to errorfill

Following my mail on the matplotlib user mailing list about fill_between and the reply from Eric (http://matplotlib.1069221.n5.nabble.com/strange-behaviour-with-fill-between-td38670.html), I have added in 'errorfill' the keyword 'where' that is passed to ax.fill_between and ax.fill_betweenx. ax.plot is drawn piecewise like fill_between.
My scope is to avoid strange behaviour in fill_between where y+-yerr <= 0 and a logarithmic y axis is used.