Citeproc: Fix bug in metaValueToReference.

This bug caused us to get some repeated content when converting
MetaBlock to Inlines.  Closes #8611.

test/command/8611.md

@@ -0,0 +1,34 @@
+```
+% pandoc -f markdown -s -t bibtex
+---
+nocite: "[@*]"
+references:
+- id: mcdowell:why
+  title: |
+    Why is Sellars's Essay Called "Empiricism and the Philosophy of Mind"?
+  author:
+  - family: McDowell
+    given: John
+  journal: Proceedings of the Aristotelian Society
+  type: chapter
+  container-title: 'Empiricism, Perceptual Knowledge, Normativity, and Realism: Essays on Wilfrid Sellars'
+  editor:
+    - family: deVries
+      given: Willem A.
+  issued: 2009
+  publisher: Oxford University Press
+  publisher-place: Oxford
+...
+^D
+@incollection{mcdowell:why,
+  author = {McDowell, John},
+  editor = {deVries, Willem A.},
+  publisher = {Oxford University Press},
+  title = {Why Is {Sellars’s} {Essay} {Called} “{Empiricism} and the
+    {Philosophy} of {Mind}”?},
+  booktitle = {Empiricism, Perceptual Knowledge, Normativity, and
+    Realism: Essays on Wilfrid Sellars},
+  year = {2009},
+  address = {Oxford}
+}
+```