Fix can't modify frozen Hash error.

Fix: #411

The load path cache can be mutated, so we need to unfreeze the hash
when it happens.

lib/bootsnap/load_path_cache/store.rb

@@ -29,8 +29,8 @@ def fetch(key)
 
         v = get(key)
         unless v
-          @dirty = true
           v = yield
+          mark_for_mutation!
           @data[key] = v
         end
         v
@@ -40,7 +40,7 @@ def set(key, value)
         raise(SetOutsideTransactionNotAllowed) unless @txn_mutex.owned?
 
         if value != @data[key]
-          @dirty = true
+          mark_for_mutation!
           @data[key] = value
         end
       end
@@ -59,6 +59,11 @@ def transaction
 
       private
 
+      def mark_for_mutation!
+        @dirty = true
+        @data = @data.dup if @data.frozen?
+      end
+
       def commit_transaction
         if @dirty
           dump_data

test/load_path_cache/store_test.rb

@@ -37,6 +37,12 @@ def test_modification
         assert_equal("c", store3.get("a"))
       end
 
+      def test_modification_of_loaded_store
+        store.transaction { store.set("a", "b") }
+        store = Store.new(@path)
+        store.transaction { store.set("c", "d") }
+      end
+
       def test_stores_arrays
         store.transaction { store.set("a", [1234, %w(a b)]) }