message: [bugfix] Picker bottom area not dragable
diff: <template>
  - <div class="van-picker-column" :class="className">
  + <div class="van-picker-column" :class="className" :style="columnStyle">
  <div class="van-picker-column__frame van-hairline--top-bottom" :style="frameStyle" />
  <ul
  :style="wrapperStyle"
  @@ -95,14 +95,18 @@ export default create({
  return this.options.length;
  },
  + columnStyle() {
  + return {
  + height: (this.itemHeight * this.visibileColumnCount) + 'px'
  + };
  + },
  +
  wrapperStyle() {
  - const { itemHeight, visibileColumnCount } = this;
  return {
  transition: `${this.duration}ms`,
  transform: `translate3d(0, ${this.offset}px, 0)`,
  - lineHeight: itemHeight + 'px',
  - height: itemHeight * visibileColumnCount + 'px',
  - paddingTop: itemHeight * (visibileColumnCount - 1) / 2 + 'px'
  + lineHeight: this.itemHeight + 'px',
  + padding: `${this.itemHeight * (this.visibileColumnCount - 1) / 2}px 0`
  };
  },
  