Generate row meta for header rows

[ahmacleod]

Generates and passes rowMeta objects for header rows. These already exist for body and footer rows, but were missing for the header.

This allows the template to access the row index for header rows, e.g.:

<EmberTable as |t|>
  <t.head @columns={{this.columns}} as |h|>
    <h.row as |r|>
      <r.cell as |column columnMeta|>
        {{#if (eq r.rowMeta.index 0)}}
          <div class="first-row-only">...</div>
        {{/if}}
        {{column.name}}
      </r.cell>
    </h.row>
  </t.head>

  {{!-- ... --}}

</EmberTable>