Fix JSON serialization in PNG compression and fixed tests

[electronicofive]

Fix JSON serialization in PNG compression #856

Problem

• pngcompression.encode() returns bytes object
• bytes objects are not JSON serializable
• Causes TypeError: Object of type bytes is not JSON serializable when requesting robot maps via rosbridge

Solution

Minimal one-line change in encode() function:

return encoded.decode()  # Convert bytes to string

Test Updates

• Fixed inconsistency between str(bytes()) and str(bytearray())
• Added assertIsInstance(encoded, str) to prevent future regressions