Fix handling of unsigned char strings (#373)

(cherry picked from commit a79c7b4e8f)

test/format-test.cc

@@ -1343,6 +1343,8 @@ TEST(FormatterTest, FormatUCharString) {
   EXPECT_EQ("test", format("{0:s}", str));
   const unsigned char *const_str = str;
   EXPECT_EQ("test", format("{0:s}", const_str));
+  unsigned char *ptr = str;
+  EXPECT_EQ("test", format("{0:s}", ptr));
 }
 
 TEST(FormatterTest, FormatPointer) {