Fail to unpack messages with custom headers in case java(pack)-->python(unpack)

[rodolfomiranda]

If a java client packs a message that includes a custom header, when it is unpacked by Python client, it fails with MalformedMessageCode.INVALID_PLAINTEXT error.

The other cases works fine: java-->java, python-->python, python-->java. All are unpacked without error.

How to reproduce the error:

• add .customHeader("return_route","all") in line 109 of DIDCommDemoHelper.kt

val message = Message.builder(
            id = UUID.randomUUID().toString(),
            body = mapOf("msg" to data),
            type = "my-protocol/1.0"
)
.customHeader("return_route","all")
.build()

• rebuild
• run demo.py
• check error at the end:
  Bob received MalformedMessageCode.INVALID_PLAINTEXT.

Same results were obtained using both libraries but outside this demo, just to confirm that is not a problem of the demo code. I'm opening the issue in this repo because I can't say if it's a problem in the python library or in the JVM library.

[dhh1128]

tagging @dmitry-vychikov and @dkulic, so you at least see this issue. It may be relevant for some of your current work because at least the java DIDComm v2 libraries are likely to be relevant, and the error may be on the java side?