Use Object.assign to ensure opts isn't mutated

[jimpick]

Some users may re-use the opts object passed as a parameter, which results in surprising behaviour protocol errors after multiple calls to .replicate() eg. random errors such as:

Only 128 feeds currently supported. Remote sent invalid feed message Remote message is larger than 8MB (max allowed)

There was also an instance of the opts object in the constructor being mutated as well, so added an Object.assign for that as well.

[jimpick]

Here was the original gist from the #dat irc channel that exhibited the issue:

https://gist.github.com/fsteff/62afca979806bfbca1ced9bd3b0d7563

[jimpick]

@reconbot Object.freeze() is a great idea. I just tried it, but it seems it only throws if I add 'use strict'; into the code. The funny thing is that without 'use strict'; - the freeze actually makes the test pass!

[mafintosh]

3.4.0 !