fix: Correct JSON serialization in PNG compression and update tests (#1044)

Co-authored-by: Juan David Vergara <electronicofive@octopusforce.com>

Author: electronicofive

rosbridge_library/src/rosbridge_library/internal/pngcompression.py

@@ -49,7 +49,7 @@ def encode(string):
     buff = BytesIO()
     i.save(buff, "png")
     encoded = standard_b64encode(buff.getvalue())
-    return encoded
+    return encoded.decode()
 
 
 def decode(string):

rosbridge_library/test/internal/test_compression.py

@@ -10,10 +10,11 @@ def test_compress(self):
         string = str(bytearray(bytes_data))
         encoded = pngcompression.encode(string)
         self.assertNotEqual(string, encoded)
+        self.assertIsInstance(encoded, str)
 
     def test_compress_decompress(self):
         bytes_data = list(range(128)) * 10000
-        string = str(bytes(bytes_data))
+        string = str(bytearray(bytes_data))
         encoded = pngcompression.encode(string)
         self.assertNotEqual(string, encoded)
         decoded = pngcompression.decode(encoded)