Honor pending spaces in InsetMathChar::write

Honor pending spaces in InsetMathChar::write

The InsetMathChar::write() method directly accesses the otexrowstream
underlying the TeXMathStream class for writing a character, thus
shortcircuiting the mechanism that allows to separate a macro from the
following material. It has to do so because directly writing a char_type
would cause printing its numerical value instead of the corresponding
unicode character in systems where char_type is typedef'd to uint32_t.

This problem has been uncovered by [7441172d/lyxgit] because each atom
of a mathed cell was being separately written to the output instead of
using the lyx::write() method in MathExtern.cpp that simply converts
everything to a docstring. As InsetMathChar::write() is the only method
bypassing the TeXMathStream machanism, it is simpler teaching it to
honor the pending space instead of modifying the code in InsetMathColor.

This commit amends 7441172d.