Correct the argument types of the StringBuilder.append

Modified AsmUtil to generate calls to more specific append methods
in the StringBuilder class in order to save computation time and make
less temporary objects.

Also adds unit-test to verify that the append(Object) method was
invoked 0 times while the append(String) 3 times

Signed-off-by: Laszlo Hornyak <laszlo.hornyak@gmail.com>