Issue #3773: Error in acos(x).rewrite(atan)

Fixed acos(x).rewrite(atan) so that it accepts symbolic inputs.
Issue #3773: Added test case to check acos(x).rewrite(atan)

Added a test case to test_trigonometric to make sure
acos(x).rewrite(atan) produces the correct formula 2*atan(sqrt(1 -
x**2)/(1 + x))