use explicit json content-type to avoid security issues with text/html

app/coffee/CompileController.coffee

@@ -84,7 +84,7 @@ module.exports = CompileController =
 		user_id = req.params.user_id
 		CompileManager.syncFromCode project_id, user_id, file, line, column, (error, pdfPositions) ->
 			return next(error) if error?
-			res.send JSON.stringify {
+			res.json {
 				pdf: pdfPositions
 			}
 
@@ -96,7 +96,7 @@ module.exports = CompileController =
 		user_id = req.params.user_id
 		CompileManager.syncFromPdf project_id, user_id, page, h, v, (error, codePositions) ->
 			return next(error) if error?
-			res.send JSON.stringify {
+			res.json {
 				code: codePositions
 			}
 
@@ -109,7 +109,7 @@ module.exports = CompileController =
 
 		CompileManager.wordcount project_id, user_id, file, image, (error, result) ->
 			return next(error) if error?
-			res.send JSON.stringify {
+			res.json {
 				texcount: result
 			}