Only run autosize only if v-autosize attribute is truthy.

[benosman]

I use this plugin inside a wrapper component, and I have a boolean prop that specifies whether autosize should be enabled/disabled.

I pass this prop through on the v-autosize attribute, however, if the value is false, the autosize functionality is still enabled as currently it only checks for the existence of the attribute not if it is 'truthy'

This commit changes this, so v-autosize="false" works as expected.