LaTeX writer: don't escape _ in url.

src/Text/Pandoc/Writers/LaTeX.hs

@@ -189,7 +189,7 @@ stringToLaTeX  isUrl (x:xs) = do
        '$' -> "\\$" ++ rest
        '%' -> "\\%" ++ rest
        '&' -> "\\&" ++ rest
-       '_' -> "\\_" ++ rest
+       '_' | not isUrl -> "\\_" ++ rest
        '#' -> "\\#" ++ rest
        '-' -> case xs of   -- prevent adjacent hyphens from forming ligatures
                    ('-':_) -> "-{}" ++ rest

tests/writer.latex

@@ -786,7 +786,7 @@ Just a \href{/url/}{URL}.
 
 \href{/url/}{URL and title}
 
-\href{/url/with\_underscore}{with\_underscore}
+\href{/url/with_underscore}{with\_underscore}
 
 \href{mailto:nobody@nowhere.net}{Email link}