Mark adapter's `storagePolicy()` as `const`

src/ArduinoJson/Strings/Adapters/FlashString.hpp

@@ -61,7 +61,7 @@ class FlashString {
     ::memcpy_P(p, s._str, n);
   }
 
-  StringStoragePolicy::Copy storagePolicy() {
+  StringStoragePolicy::Copy storagePolicy() const {
     return StringStoragePolicy::Copy();
   }
 

src/ArduinoJson/Strings/Adapters/JsonString.hpp

@@ -15,7 +15,7 @@ class JsonStringAdapter : public SizedRamString {
   JsonStringAdapter(const JsonString& s)
       : SizedRamString(s.c_str(), s.size()), _linked(s.isLinked()) {}
 
-  StringStoragePolicy::LinkOrCopy storagePolicy() {
+  StringStoragePolicy::LinkOrCopy storagePolicy() const {
     StringStoragePolicy::LinkOrCopy policy = {_linked};
     return policy;
   }

src/ArduinoJson/Strings/Adapters/RamString.hpp

@@ -53,7 +53,7 @@ class ZeroTerminatedRamString {
     return stringCompare(a, b) == 0;
   }
 
-  StringStoragePolicy::Copy storagePolicy() {
+  StringStoragePolicy::Copy storagePolicy() const {
     return StringStoragePolicy::Copy();
   }
 
@@ -83,7 +83,7 @@ class StaticStringAdapter : public ZeroTerminatedRamString {
  public:
   StaticStringAdapter(const char* str) : ZeroTerminatedRamString(str) {}
 
-  StringStoragePolicy::Link storagePolicy() {
+  StringStoragePolicy::Link storagePolicy() const {
     return StringStoragePolicy::Link();
   }
 };
@@ -121,7 +121,7 @@ class SizedRamString {
     return _str;
   }
 
-  StringStoragePolicy::Copy storagePolicy() {
+  StringStoragePolicy::Copy storagePolicy() const {
     return StringStoragePolicy::Copy();
   }