Fix instanceof checks with constructor name comparison

[marcos2872]

applying getPosition corrections when using pie chart

The problem occurs at the getPositioner function.

function getPositioner(el) {
    if (el instanceof chart_js.ArcElement) {
      return positioners.arc;
    }
    if (el instanceof chart_js.PointElement) {
      return positioners.point;
    }
    if (el instanceof chart_js.BarElement) {
      return positioners.bar;
    }
    return positioners.fallback;
  }

In this function when using Pie or Doughnut charts at the first if should return but it happens that the condition el instanceof chart_js.ArcElement isn't evaluated to true, because chart_js (that is require(chart.js)) is not equal to the chart.js which calling this plugin.

As a workaround I rewrite that function as below and it works fine.

function getPositioner(el) {
    if (el.constructor.name === 'ArcElement') {
      return positioners.arc;
    }
    if (el.constructor.name === 'PointElement') {
      return positioners.point;
    }
    if (el.constructor.name === 'BarElement') {
      return positioners.bar;
    }
    return positioners.fallback;
  }

Reference: #141