http_parser: assert on incoming argument's type

Unchecked argument conversion in Parser::Consume crashes node in an slightly undesirable manner - 'unreachable code' in parser.

Make sure we validate the incoming type at the earliest point.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

http_parser