scanner: use infinitive verb after auxiliary word could

Could, as well as should, shall, must, may, can, might, etc.
are auxiliary words. After an auxiliary word should come an
infinitive verb.

tests/data/invalid-uri.loader-error

diff --git a/tests/data/invalid-uri.loader-error b/tests/data/invalid-uri.loader-error
new file mode 100644
index 0000000..06307e0
--- /dev/null
+++ b/tests/data/invalid-uri.loader-error
@@ -0,0 +1 @@
+--- !foo!   bar