Fixed metadata options in create_corner_cases_zim_file

Specifying the = symbol with single-character options makes that
character included in the option value (e.g. -l=en results in the
language of the ZIM file being set to =en).

test/data/create_corner_cases_zim_file

@@ -5,11 +5,11 @@ rm -f corner_cases.zim
 zimwriterfs --withoutFTIndex --dont-check-arguments \
             -w empty.html \
             -I empty.png \
-            -l=en \
-            -t="ZIM corner cases" \
-            -d="" \
-            -c="" \
-            -p="" \
+            -l en \
+            -t "ZIM corner cases" \
+            -d "" \
+            -c "" \
+            -p "" \
             corner_cases \
             corner_cases.zim \
 && echo 'corner_cases.zim was successfully created' \