We're reading a byte (8 bits unsigned), not a char (8 bits signed) when we peek

@JasonL9000: Seem reasonable to you.

Fixes:
```
stig/server/server.cc:1904:19: error: comparison of constant 128 with expression of type 'char' is always true [-Werror,-Wtautological-constant-out-of-range-compare]
    if (in.Peek() != Mynde::BinaryMagicRequest) {
        ~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~
```