  Consecutive proton irradiation of iron pnictides reduces the superconducting
transition temperature to decrease monotonically. Our systematic optical THz
investigations of Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ thin films reveal,
however, that the low-energy superconducting gap is first suppressed, but
recovers for higher irradiation doses. At the same time the decrease of the
superfluid density with disorder comes to a halt. These observations are in
excellent agreement with theoretical calculations, which predict a
disorder-induced transition of the order parameter from $s_{\pm}$ to $s_{++}$
symmetry. Thereby we solve the longstanding puzzle of the iron pnictides gap
symmetry in favor of $s_{\pm}$-symmetry and demonstrate THz spectroscopy as a
tool to determine the symmetry of the superconducting order parameter.
