Prevent R warnings during patchstack-mail filtering

Due to using the wrong indexing operator for a data.frame column, the warning
`In xtfrm.data.frame(x) : cannot xtfrm data frames` was printed during the
filtering of patchstack mails. This is fixed by using the `[[` operator
instead of the `[` operator to access the column.

Signed-off-by: Thomas Bock <bockthom@cs.uni-saarland.de>

util-data.R

@@ -269,7 +269,7 @@ ProjectData = R6::R6Class("ProjectData",
             result = parallel::mclapply(thread.data, function(thread) {
 
                 ## ensure that all mails within the thread are ordered correctly
-                thread = thread[order(thread["date"]), ]
+                thread = thread[order(thread[["date"]]), ]
 
                 running = TRUE
                 i = 1