sort: fix -k2M (wasn't skipping leading whitespace)

function                                             old     new   delta
compare_keys                                         848     862     +14

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/testsuite/sort.tests b/testsuite/sort.tests
index ff33e21..fb2cc91 100755
--- a/testsuite/sort.tests
+++ b/testsuite/sort.tests
@@ -219,4 +219,15 @@
 
 # testing "description" "command(s)" "result" "infile" "stdin"
 
+testing "sort -k2,2M" \
+"sort -k2,2M input" "\
+3 March
+2 April
+1  May
+" "\
+2 April
+1  May
+3 March
+" ""
+
 exit $FAILCOUNT