fix(httpParamSerializerJQLike): Follow jQuery for null and undefined

[lgalfaso]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Follow jQuery when serializing null and undefined.

What is the current behavior? (You can also link to an open issue here)

$httpParamSerializerJQLike({items:['foo', 'bar', null, undefined]}) ==
  'items%5B%5D=foo&items%5B%5D=bar';

What is the new behavior (if this is a feature change)?

$httpParamSerializerJQLike({items:['foo', 'bar', null, undefined]}) ==
  'items%5B%5D=foo&items%5B%5D=bar&items%5B%5D=&items%5B%5D=';

Does this PR introduce a breaking change?
Yes, but does so to follow the implementation in jQuery.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

Closes: #15969

[gkalpak]

LGTM, but I lean towards not backporting to 1.6. due to the breaking change (even if only to follow jQuery).

[lgalfaso]

Merged into master as 301fdda
Agree in not back porting this into 1.6.x