Fixed "method should not be called statically" errors

Author: jaaufauvre

src/Developer/Encryption/FieldLevelEncryption.php

@@ -56,7 +56,7 @@ public static function encryptPayload($payload, $config, $params = null) {
      * @return The updated payload
      * @throws EncryptionException
      */
-    public function decryptPayload($payload, $config, $params = null) {
+    public static function decryptPayload($payload, $config, $params = null) {
         try {
             // Parse the given payload
             $payloadJsonObject = json_decode($payload);

src/Developer/Encryption/FieldLevelEncryptionParams.php

@@ -175,7 +175,7 @@ private static function toDsigXmlPrivateKey($raw) {
     /**
      * @throws EncryptionException
      */
-    private function getOrComputeEncryptionCertificateFingerprint($config) {
+    private static function getOrComputeEncryptionCertificateFingerprint($config) {
         try {
             $providedCertificateFingerprintValue = $config->encryptionCertificateFingerprint;
             if (!empty($providedCertificateFingerprintValue)) {

src/Developer/Utils/StringUtils.php

@@ -3,11 +3,11 @@
 
 class StringUtils {
 
-    public function startsWith($haystack, $needle) {
+    public static function startsWith($haystack, $needle) {
         return empty($needle) || strrpos($haystack, $needle, -strlen($haystack)) !== false;
     }
 
-    public function endsWith($haystack, $needle) {
+    public static function endsWith($haystack, $needle) {
         if (empty($needle)) {
             return true;
         }