config: add failing test for preserving case when writing keys

While most parts of a configuration key are case-insensitive, we should still be
case-preserving and write down whatever string the caller provided.

tests/config/write.c

@@ -722,3 +722,26 @@ void test_config_write__repeated(void)
 
 	git_config_free(cfg);
 }
+
+void test_config_write__preserve_case(void)
+{
+	const char *filename = "config-preserve-case";
+	git_config *cfg;
+	git_buf result = GIT_BUF_INIT;
+	const char *expected = "[sOMe]\n" \
+		"\tThInG = foo\n" \
+		"\tOtheR = thing\n";
+
+	cl_git_pass(git_config_open_ondisk(&cfg, filename));
+	cl_git_pass(git_config_set_string(cfg, "sOMe.ThInG", "foo"));
+	cl_git_pass(git_config_set_string(cfg, "SomE.OtheR", "thing"));
+	git_config_free(cfg);
+
+	cl_git_pass(git_config_open_ondisk(&cfg, filename));
+
+	cl_git_pass(git_futils_readbuffer(&result, filename));
+	cl_assert_equal_s(expected, result.ptr);
+	git_buf_free(&result);
+
+	git_config_free(cfg);
+}