harmonicOn doesn't work in TabStaff

[Federico Bruni]

As you can see in the following example, when using harmonicOn, fret
numbers in TabStaff are not enclosed by angled brackets (while Staff is
ok).

It looks like a bug.

\version "2.15.0"

music = \relative c'' {
   % Ok, it works in Staff and TabStaff
  <b\2\harmonic>2 <e\harmonic>

  % it works on Staff only: bug or what?
  \harmonicsOn
  b2\2 e
  \harmonicsOff

  <g,\3\harmonic>1
}

\new StaffGroup <<
  \new Staff { \clef "G_8" \music }
  \new TabStaff { \clef "moderntab" \music }
>>