Don't update value if tearing component down

[Robdel12]

Related to #94

We don't want to set the value of the component if we're destroying the component. This is what's happening in #94.

TODOs:

• Write test to prevent this from happening again
• Verify with @fivetanley / @cowboyd this is the right solution.

[Robdel12]

Also should this check if multiple=true is set and do this._updateValueMultiple()?

if (!this.get('isDestroying')) {
  if (this.get('multiple')) {
    this._updateValueMultiple();
  } else {
    this.updateValueSingle();
  }
} 

[jevanlingen]

@Robdel12: Yeah you should check that! Just wanted to add a PR to fix the multiple=true case. Without the fix the value will change from an array to one single value, as soon as the component tears down.

Btw just use this._setDefaultValues() instead of this._updateValueSingle(), no need to write duplicate code.