More consistent, broader compatible Dataset access API

[dseeker]

The problem occurs in the following line

weex-vue-render/src/utils/lazyload.js

Line 138 in 21b2c6b

let id = +(el && el.dataset.throttleId)

Currently the dataset API is used on this line, however just a few lines below the setAttribute method is also used.
el && el.setAttribute('data-throttle-id', id + '')

On some browsers using the dataset api results in mount errors like this:

START WEEX VUE RENDER: 1.0.36, Build 2018-12-29 17:52. 
[Vue Render] install Vue 2.6.10. 
[Vue warn]: Error in beforeCreate hook: "TypeError: Unable to get property 'throttleId' of undefined or null reference" (found in <Root>) 
TypeError: Unable to get property 'throttleId' of undefined or null reference 
[Vue warn]: Error in mounted hook: "TypeError: Unable to get property 'throttleId' of undefined or null reference" (found in <Root>) 
TypeError: Unable to get property 'throttleId' of undefined or null reference 

I suggest replacing dataset with getAttribute method to be more universally compatible and consistent.

from let id = +(el && el.dataset.throttleId)
to let id = +(el && el.getAttribute('data-throttle-id'))