Text.Pandoc.Parsing.uri: allow & and = as word characters.

This fixes a bug where pandoc would stop parsing a URI with an
empty attribute:  for example, `&a=&b=` wolud stop at `a`.
(The uri parser tries to guess which punctuation characters
are part of the URI and which might be punctuation after it.)

Closes #4068.

src/Text/Pandoc/Parsing.hs

@@ -563,7 +563,7 @@ uri = try $ do
   -- http://en.wikipedia.org/wiki/State_of_emergency_(disambiguation)
   -- as a URL, while NOT picking up the closing paren in
   -- (http://wikipedia.org). So we include balanced parens in the URL.
-  let isWordChar c = isAlphaNum c || c `elem` "#$%*+/@\\_-"
+  let isWordChar c = isAlphaNum c || c `elem` "#$%*+/@\\_-&="
   let wordChar = satisfy isWordChar
   let percentEscaped = try $ char '%' >> skipMany1 (satisfy isHexDigit)
   let entity = () <$ characterReference

test/command/4068.md

@@ -0,0 +1,9 @@
+```
+pandoc -f mediawiki -t native
+[https://domain.com/script.php?a=1&b=2&c=&d=4 open productname bugs]
+
+[http://domain.com?a=. open productname bugs]
+^D
+[Para [Link ("",[],[]) [Str "open",Space,Str "productname",Space,Str "bugs"] ("https://domain.com/script.php?a=1&b=2&c=&d=4","")]
+,Para [Str "[",Link ("",[],[]) [Str "http://domain.com?a="] ("http://domain.com?a=",""),Str ".",Space,Str "open",Space,Str "productname",Space,Str "bugs]"]]
+```