Implement md.merge(other)

[asvetlov]

The method should be mimic to md.extend() but add only keys from other that are absent in self.

The behavior is very useful for merging aiohttp session defaults into passed headers for example.

[webknjaz]

Why not replace existing keys?

[asvetlov]

Replacing is another useful strategy already implemented by md.update().
I'm talking about this case:

1. Aiohttp client session has a set of default HTTP headers.
2. client.request() accepts headers for the concrete request.
   Default headers should be merged into passed to request() one but session defaults should not override formers.

[webknjaz]

Ok, makes sense