GWRequestHandler: prefer data() over c_str()
GWRequestHandler: prefer data() over c_str()

The data() returns raw pointer to the underlying string.
The c_str() must always ensure that the string ends with '\0'
which does not have to be true for data() and thus the c_str()
method might be slightely more complex and less stable.

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server