Change DateOffset from Z to +00:00

I missed that getDateTimeOffsetNow is extracting the last 5 chars
for DateTimeOffset. So this patch changes the offset to the original
"+00:00" one.

Tested:
1. unit tests
2. Redfish Validator Tests: no errors found on DateTime or
DateTimeLocalOffset.
```
DateTime	1970-01-01T00:13:27+00:00	date	Yes	PASS
DateTimeLocalOffset	+00:00	string	Yes	PASS
```
All other errors are not related.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I24977c476f18c88515d759e278ec56e5cbb73b3a
diff --git a/http/ut/utility_test.cpp b/http/ut/utility_test.cpp
index fc8b90e..cb914d5 100644
--- a/http/ut/utility_test.cpp
+++ b/http/ut/utility_test.cpp
@@ -61,17 +61,17 @@
 {
     // some time before the epoch
     EXPECT_EQ(crow::utility::getDateTimeStdtime(std::time_t{-1234567}),
-              "1969-12-17T17:03:53Z");
+              "1969-12-17T17:03:53+00:00");
     // epoch
     EXPECT_EQ(crow::utility::getDateTimeStdtime(std::time_t{0}),
-              "1970-01-01T00:00:00Z");
+              "1970-01-01T00:00:00+00:00");
     // some time in the past after the epoch
     EXPECT_EQ(crow::utility::getDateTimeUint(uint64_t{1638312095}),
-              "2021-11-30T22:41:35Z");
+              "2021-11-30T22:41:35+00:00");
     // some time in the future, beyond 2038
     EXPECT_EQ(crow::utility::getDateTimeUint(uint64_t{41638312095}),
-              "3289-06-18T21:48:15Z");
+              "3289-06-18T21:48:15+00:00");
     // the maximum time we support
     EXPECT_EQ(crow::utility::getDateTimeUint(uint64_t{253402300799}),
-              "9999-12-31T23:59:59Z");
+              "9999-12-31T23:59:59+00:00");
 }
diff --git a/http/utility.hpp b/http/utility.hpp
index d141056..033755e 100644
--- a/http/utility.hpp
+++ b/http/utility.hpp
@@ -584,7 +584,7 @@
     boost::posix_time::ptime epoch(boost::gregorian::date(1970, 1, 1));
     boost::posix_time::ptime time = epoch + secondsSinceEpoch;
     // append zero offset to the end according to the Redfish spec for Date-Time
-    return boost::posix_time::to_iso_extended_string(time) + 'Z';
+    return boost::posix_time::to_iso_extended_string(time) + "+00:00";
 }
 
 inline std::string getDateTimeUint(uint64_t secondsSinceEpoch)