fix(MdTable): remove table head while column removed

[VdustR]

compute MdTable.items again on column remove.

fix #1347

[VdustR]

My test case:

<template>
  <div>
    <div>
      <md-checkbox v-model="idShown">ID</md-checkbox>
      <md-checkbox v-model="nameShown">Name</md-checkbox>
      <md-checkbox v-model="emailShown">Email</md-checkbox>
      <md-checkbox v-model="genderShown">Gender</md-checkbox>
      <md-checkbox v-model="jobShown">Job Title</md-checkbox>
    </div>
    <div>
      <md-checkbox v-model="show[0]">1</md-checkbox>
      <md-checkbox v-model="show[1]">2</md-checkbox>
      <md-checkbox v-model="show[2]">3</md-checkbox>
      <md-checkbox v-model="show[3]">4</md-checkbox>
      <md-checkbox v-model="show[4]">5</md-checkbox>
    </div>
    <md-table v-model="usersToShow" md-card>
      <md-table-toolbar>
        <h1 class="md-title">Users</h1>
      </md-table-toolbar>

      <md-table-row slot="md-table-row" slot-scope="{ item }">
        <md-table-cell v-if="idShown" md-label="ID" md-sort-by="id" md-numeric>{{ item.id }}</md-table-cell>
        <md-table-cell v-if="nameShown" md-label="Name" md-sort-by="name">{{ item.name }}</md-table-cell>
        <md-table-cell v-if="emailShown" md-label="Email" md-sort-by="email">{{ item.email }}</md-table-cell>
        <md-table-cell v-if="genderShown" md-label="Gender" md-sort-by="gender">{{ item.gender }}</md-table-cell>
        <md-table-cell v-if="jobShown" md-label="Job Title" md-sort-by="title">{{ item.title }}</md-table-cell>
      </md-table-row>
    </md-table>
  </div>
</template>

<script>
  export default {
    name: 'TableTemplate',
    data: () => ({
      idShown: false,
      nameShown: false,
      emailShown: false,
      genderShown: false,
      jobShown: false,
      show: [false, false, false, false, false],
      users: [
        {
          id: 1,
          name: 'Shawna Dubbin',
          email: 'sdubbin0@geocities.com',
          gender: 'Male',
          title: 'Assistant Media Planner'
        },
        {
          id: 2,
          name: 'Odette Demageard',
          email: 'odemageard1@spotify.com',
          gender: 'Female',
          title: 'Account Coordinator'
        },
        {
          id: 3,
          name: 'Lonnie Izkovitz',
          email: 'lizkovitz3@youtu.be',
          gender: 'Female',
          title: 'Operator'
        },
        {
          id: 4,
          name: 'Thatcher Stave',
          email: 'tstave4@reference.com',
          gender: 'Male',
          title: 'Software Test Engineer III'
        },
        {
          id: 5,
          name: 'Clarinda Marieton',
          email: 'cmarietonh@theatlantic.com',
          gender: 'Female',
          title: 'Paralegal'
        }
      ]
    }),
    computed: {
      usersToShow () {
        return this.users.filter((user, index) => this.show[index])
      }
    }
  }
</script>