Solr backend: correct usage of pysolr delete
Solr backend: correct usage of pysolr delete

We use HAYSTACK_ID_FIELD in other places but the value passed to
pysolr's delete() method must use the keyword argument ``id``:

https://github.com/toastdriven/pysolr/blob/v3.1.0/pysolr.py#L756

Although the value is passed to Solr an XML tag named ``<id>`` it will
always be checked against the actual ``uniqueKey`` field even if it uses
a custom name:

https://wiki.apache.org/solr/UpdateXmlMessages#A.22delete.22_documents_by_ID_and_by_Query

Closes #943

streeter/django-haystack,kuanyui/django-haystack,fisle/django-haystack,vitalyvolkov/django-haystack,blancltd/django-haystack,sgaist/django-haystack,jannon/django-haystack,askaliuk/django-haystack,kybi/django-haystack,Stupeflix/django-haystack,antonyr/django-haystack,zeehio/django-haystack,streeter/django-haystack,jannon/django-haystack,kybi/django-haystack,sgaist/django-haystack,elishowk/django-haystack,antonyr/django-haystack,jannon/django-haystack,askaliuk/django-haystack,cyanut/django-haystack,fisle/django-haystack,django-searchstack/django-searchstack,speedplane/django-haystack,askaliuk/django-haystack,barseghyanartur/django-haystack,eventials/django-haystack,ruimashita/django-haystack,ruimashita/django-haystack,django-searchstack/django-searchstack,barseghyanartur/django-haystack,barseghyanartur/django-haystack,kuanyui/django-haystack,dionysio/django-haystack,django-searchstack/django-searchstack,blancltd/django-haystack,streeter/django-haystack,zeehio/django-haystack,eventials/django-haystack,elishowk/django-haystack,sgaist/django-haystack,comandrei/django-haystack,Stupeflix/django-haystack,eventials/django-haystack,celerityweb/django-haystack,cyanut/django-haystack,fisle/django-haystack,vitalyvolkov/django-haystack,speedplane/django-haystack,comandrei/django-haystack,speedplane/django-haystack,celerityweb/django-haystack,elishowk/django-haystack,vitalyvolkov/django-haystack,dionysio/django-haystack,cyanut/django-haystack,celerityweb/django-haystack,kuanyui/django-haystack,Stupeflix/django-haystack,zeehio/django-haystack,antonyr/django-haystack,blancltd/django-haystack,dionysio/django-haystack,ruimashita/django-haystack,comandrei/django-haystack