Do not export the content of InsetMathBoxed as text in MathML

\boxed{} is the only box in LaTeX that contains math. It is amusing
that the example file that we had for ticket #13609 should not have
been a problem since there is no reason to force <mtext> there.

(cherry picked from commit 1186d90edf)

src/mathed/InsetMathBox.cpp

@@ -402,7 +402,9 @@ void InsetMathBoxed::infoize(odocstream & os) const
 
 void InsetMathBoxed::mathmlize(MathMLStream & ms) const
 {
-	mathmlizeHelper(ms, cell(0), "class='boxed'");
+	ms << MTag("mrow", "class='boxed'");
+	ms << cell(0);
+	ms << ETag("mrow");
 }
 
 

status.24x

@@ -49,6 +49,8 @@ What's new
 
 * DOCUMENT INPUT/OUTPUT
 
+- When exporting to MathML, the argument of the \boxed{} macro is now
+  correctly treated as math.
 
 
 * USER INTERFACE