Avoid -Winvalid-source-encoding warnings

[thatsafunnyname]

Hello and thanks for fastcgipp, Compiling with clang++ v5 produced -Werror,-Winvalid-source-encoding warnings as errors on these characters, using the hex code avoids the warning.

fastcgipp-3.0/examples/sessions.cpp:105:57: error: illegal character encoding in string literal
      [-Werror,-Winvalid-source-encoding]
                                "Text: <input type='text' name='data' value='Hola se<F1>or, usted "
                                                                                    ^~~~
fastcgipp-3.0/examples/sessions.cpp:106:32: error: illegal character encoding in string literal
      [-Werror,-Winvalid-source-encoding]
                    "me almacen<F3> en una sesi<F3>n' />"
                               ^~~~            ~~~~
2 errors generated.

[eddic]

Perfect. This was messing up rendering for the doxygen example so this fixes that as well. Thanks.