Finding a transfer function by voltage division
If you apply the current divider formula, and observe the direction of \$i_2\$, the result is: 

\$T(s) = -\frac {Z_T}{Z_2}\$ where 

\$Z_T =\frac{R_1(R_2+sL)}{R_1+R_2+sL}\$ and 

\$Z_2 = R_2 + sL\$

so 

\$T(s) = -\frac {R_1}{R_1+R_2+sL}\$

The negative sign only arises because \$i_2\$ is defined in the opposite direction to \$i_1\$.