[whatwg] HTMLTableElement should have a createTBody method

[Adam Roben]

    The omission of a createTBody method from HTMLTableElement makes  
it rather inconvenient to create a table with both a thead and a tbody  
using the table DOM APIs. After creating a thead, you have to manually  
create and append the tbody to start putting rows into the body of the  
table, so you cannot exclusively use the HTMLTableElement methods to  
populate the table. If a createTBody method is added, I'd suggest that  
it always create a new tbody rather than ever returning an existing one.

-Adam