The option `socket_opts` in Server accepts multiple values

[xinguanz]

Is your feature request related to a problem? Please describe.
The current cowboy adapter implementation in Server accepts only one customized value as socket_opts.
The option with the key :net will be treated as socket_opts.
It was fine since there could be multiple keywords with the same key :net.
However the opts were merged into a Map in earlier steps, which means that only one customized socket_opts option can be accepted.

Describe the solution you'd like
Multiple customized socket_opts are accepted.

[sleipnir]

Closed by #431