Fix Promise.map unhandled rejections (#1487)

[overlookmotel]

This PR fixes #1487.

Previously, unhandled rejection errors can occur with Promise.map(), Promise.filter(), .map() and .filter() when in fact the rejections are handled.

Prior to this PR, all of the following create an unhandled rejection, after this PR they do not.

var p = Promise.reject( new Error('foo') );
Promise.map( p, () => {} ).catch( () => {} );

var arr = [ Promise.reject( new Error('foo') ) ];
Promise.map( arr, () => {} ).catch( () => {} );

var p = Promise.reject( new Error('foo') );
p.map( () => {} ).catch( () => {} );

var arr = [ Promise.reject( new Error('foo') ) ];
var p = Promise.resolve( arr );
p.map( () => {} ).catch( () => {} );


var p = Promise.reject( new Error('foo') );
Promise.filter( p, () => {} ).catch( () => {} );

var arr = [ Promise.reject( new Error('foo') ) ];
Promise.filter( arr, () => {} ).catch( () => {} );

var p = Promise.reject( new Error('foo') );
p.filter( () => {} ).catch( () => {} );

var arr = [ Promise.reject( new Error('foo') ) ];
var p = Promise.resolve( arr );
p.filter( () => {} ).catch( () => {} );

I have not written tests as I'm not sure how to test for unhandled rejections.

[benjamingr]

Can you add tests?

What about two rejections in a single array?

[overlookmotel]

Would be happy to write tests, but can you advise how to write a test for an unhandled rejection? By it's nature it's hard to test for.

2 rejections in a single array should also be handled as iteration over the array now happens synchronously.

[overlookmotel]

@benjamingr Tests added. I've used ._isRejectionUnhandled() to check that the rejections are handled synchronously. Seems a bit wonky to be using private undocumented APIs in tests, but I can't see any other reasonable way to do it.

[overlookmotel]

NB The test also covers case of 2 rejections in an array.