Tables in MS Word are set by default to have special first-row
formatting, which pandoc uses to determine whether or not they have a
header. This means that one-row tables will, by default, have only a
header -- which we imagine is not what people want. This change
ensures that a one-row table is not understood to be a header only.
Note that this means that it is impossible to produce a header-only
table from docx, even though it is legal pandoc. But we believe that
in nearly all cases, it will be an accidental (and unwelcome) result
Closes#3285.
