Add optional verification of root name matching

[cowtowncoder]

(follow-up for #374)

As of 2.11, Jackson does not actually verify that the root level element name matches expected name: it should. However, since there may be some usage where names do not match, this needs to be XML read feature; default could be either to validate or not (need to think about it).

[cowtowncoder]

Realized this is dup: #247 is older, closing this.