Patch: Fix return value of InsetFormula::latex

[Dekel Tsur]

I've attached a patch that fix the return value of InsetFormula::latex.
The fix is done by counting the number of \n chars in every string that is
written to WriteStream.

A more efficient solution is to check for \n only in
WriteStream::operator<<(char c), but this will make the code ugly, because
we will then need to use 
  os << "\begin{equation}" << '\n'
instead of
  os << "\begin{equation}\n"

patch.gz
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