field variable sets autocompletion index

the variable $this->field was set but not used right?
I made this change so that the field variable defines the field indexed for the autocompletion.
usage example where the uri's of all pages are used for autocompletion:

````
related:
    label: Related pages
    type: tags
    index: all
    lower: true
    field: uri
````
Fixed another possible bug
Merge branch 'mcbrwr-patch-1' into develop