[TheRealAlexxx]

Resolv: build DNS wire messages as binary strings

Fixes [Bug #19697].

Resolv::DNS::MessageEncoder accumulates the encoded packet in a UTF-8 buffer. When a query name contains non-ASCII characters, such as an internationalized domain name, the buffer becomes a UTF-8 string holding non-ASCII bytes, and binary writes into it start failing: assigning the request id in ConnectedUDP#sender (request[0,2] = ...) raises Encoding::CompatibilityError (incompatible character encodings: UTF-8 and ASCII-8BIT), so resolving such a name crashes before any packet is sent. The same hazard exists mid-encode, where put_length16 backpatches a packed length into the buffer after writing a name.

The wire format is bytes, so keep the buffer binary throughout: start it as ASCII-8BIT and append the binary form of byte strings, labels and character-strings. (An empty buffer adopts the encoding of the first string appended to it, so the initial conversion alone is not enough.) Message#encode now consistently returns an ASCII-8BIT string, matching what the socket layer and Message.decode already work with.

The new test resolves an internationalized domain name against a stub DNS server; without the change it raises Encoding::CompatibilityError in ConnectedUDP#sender.