TLS does not enforce ALPN protocol

[carl-mastrangelo]

According to https://tools.ietf.org/html/rfc7540#section-3.3 connections over TLS use the "h2" application protocol identifier. Attempting to use another protocol identifier, such as "h2c", should fail the connection. Currently, the Grpc go server accepts using this invalid identifier when establishing a TLS connection.

Here is the test that fails:

https://github.com/grpc/grpc/blob/master/tools/http2_interop/http2interop.go#L235