Fix compiler warnings about narrowing conversion

The latest 1.5.7 beta release of Arduino includes an updated toolchain which now issues warnings about narrowing of ints to chars inside curly braces becoming ill-formed in C++11 if warnings are enabled.

For example:
```
Webduino/WebServer.h:995:35: warning: narrowing conversion of 'ch1' from 'int' to 'char' inside { } is ill-formed in C++11 [-Wnarrowing]
```

The commit below explicitly casts the values to char to prevent these warnings being emitted.