Flag transfer problem in failed .include assertion

[meeber]

Proxy-related frames are showing up in the stack trace for the .include assertion when testing for a subset of properties in an object. Also, custom error messages aren't being displayed in this case.

Example:

expect({x: {a: 1}}).to.include({x: {x: 9}}, "oh noooo");

Output:

  AssertionError: expected { x: { a: 1 } } to have property 'x' of { x: 9 }, but got { a: 1 }
  + expected - actual

   {
  -  "a": 1
  +  "x": 9
   }

  at Proxy.<anonymous> (node_modules/chai/lib/chai/core/assertions.js:322:25)
  at Array.forEach (native)
  at Proxy.include (node_modules/chai/lib/chai/core/assertions.js:317:13)
  at Proxy.chainableMethodWrapper (node_modules/chai/lib/chai/utils/addChainableMethod.js:93:49)
  at Context.<anonymous> (test/index.js:8:26)

I suspect both problems are a lack of flags being transferred to the .property assertion that the .include assertion calls internally. Will investigate later, just noting for now.

[meeber]

Just realized this problem is much more widespread than .include, and predates the proxy stuff. Basically, any assertion that creates another assertion internally isn't going to filter out Chai's implementation frames correctly. This happens to be the exact same root problem as described here regarding the assert interface, so an ideal solution would fix that occurrence of the problem too.

I think expanding the usage of the already existing keep_ssfi flag will do the trick. Working on a PR.

Also, most assertions aren't always passing along a user's custom error message either, but this isn't exactly the same problem. Will solve separately.