utils: fix mcc/mnc encoding in dec_plmn (EF_PLMNsel)

The dec_plmn function takes an hexstring and returns the decoded MCC and
MNC as integer values. The result is then used by the json encoder in
EF_PLMNsel, which means the json output will contrary to the input, use
integer values instead of strings.

This is not correct since there may be leading zeros (e.g. mnc 01 and
001 both exist are different) which must be retained in order to know
the correct length of the MNC.

Related: OS#4963
Change-Id: I393e04836814d992d2a6d0a4e4e01850976d6e81
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 71c0eb0..b70b17b 100755
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -37,6 +37,12 @@
 	def testDecMCCfromPLMN_unused(self):
 		self.assertEqual(utils.dec_mcc_from_plmn("ff0f00"), 4095)
 
+	def testDecMCCfromPLMN_str(self):
+		self.assertEqual(utils.dec_mcc_from_plmn_str("92f501"), "295")
+
+	def testDecMCCfromPLMN_unused_str(self):
+		self.assertEqual(utils.dec_mcc_from_plmn_str("ff0f00"), "")
+
 	def testDecMNCfromPLMN_twoDigitMNC(self):
 		self.assertEqual(utils.dec_mnc_from_plmn("92f501"), 10)
 
@@ -46,6 +52,15 @@
 	def testDecMNCfromPLMN_unused(self):
 		self.assertEqual(utils.dec_mnc_from_plmn("00f0ff"), 4095)
 
+	def testDecMNCfromPLMN_twoDigitMNC_str(self):
+		self.assertEqual(utils.dec_mnc_from_plmn_str("92f501"), "10")
+
+	def testDecMNCfromPLMN_threeDigitMNC_str(self):
+		self.assertEqual(utils.dec_mnc_from_plmn_str("031263"), "361")
+
+	def testDecMNCfromPLMN_unused_str(self):
+		self.assertEqual(utils.dec_mnc_from_plmn_str("00f0ff"), "")
+
 	def test_enc_plmn(self):
 		with self.subTest("2-digit MCC"):
 			self.assertEqual(utils.enc_plmn("001", "01F"), "00F110")