Fix bibliography month persistence bug

The month from one entry was incorrectly persisting to subsequent entries that didn't have a month defined. This fix initializes entrymonth and entryyear variables to empty strings before checking if they exist in the entry data.

Fixes alshedivat/al-folio#3386

Author: dfuchss

_layouts/bib.liquid

@@ -168,9 +168,11 @@
     {% else %}
       {% capture entrytype %}{% endcapture %}
     {% endif %}
+    {% assign entrymonth = '' %}
     {% if entry.month %}
       {% capture entrymonth %}{{ " " }}{{ entry.month | capitalize }}{% endcapture %}
     {% endif %}
+    {% assign entryyear = '' %}
     {% if entry.year %}
       {% capture entryyear %}{{ " " }}{{ entry.year }}{% endcapture %}
     {% endif %}