fix(core): don't extend html elements

[vltansky]

Angular SSR build error:
run npm run angular:dev:ssr on master to reproduce.

ERROR RangeError: Maximum call stack size exceeded
    at Array.filter (<anonymous>)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7616:55)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7637:15)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7637:15)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7637:15)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7637:15)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7637:15)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7637:15)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7637:15)
    at extend (D:\projects\swiper\dist\demo\server\home-home-module.js:7637:15)

extend() function from src/utils.js passing object from HTMLElement type to itself as it recognizes it as Object (that is technically right). That's leading to recursion, as HTMLElement has a lot of inner objects.

This PR adds check !(nextSource instanceof HTMLElement) to extend() function to skip HTML Elements from extending.

closes #4576