HTTP/2 to HTTP/1 non-ascii headers reset stream

Motivation:
The HTTP/2 specification indicates that when converting from HTTP/2 to HTTP/1.x and non-ascii characters are detected that an error should be thrown.

Modifications:
- The ASCII validation is already done but the exception that is raised is not properly converted to a RST_STREAM error.

Result:
- If HTTP/2 to HTTP/1.x translation layer is in use and a non-ascii header is received then a RST_STREAM frame should be sent in response.
Maybe you could use this utility method to help out here: https://github.com/netty/netty/blob/master/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2CodecUtil.java#L134