Pylightning: pass through non-ascii characters in the JSON input formatting

[jarret]

Addresses issue #2753, which describes the problem and exception that was being seen.

This change enables non-ascii characters such as emojis to be included in the description field via pylightning:

Have done a bit of testing by passing in ascii strings: \ud83d\udc7d \\\\udc74 and \\udc7d seem to work as being interpreted as literal ascii rather than some unicode character, which is what is intended. I suspect the JSON encoding/decoding is handling this correctly in this mode.

[rustyrussell]

Definitely needs a test! We had an issue in the other direction, where we didn't handle RPC output properly.

But this doesn't seem to work when I tried it:

    def test_unicode_input(node_factory):
        l1 = node_factory.get_node()
    
        for s in [ '\ud83d\udc7d', '\\\\udc74', '\\udc7d' ]:
>           inv = l1.rpc.invoice(123000, s, s)

tests/test_invoices.py:498: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
contrib/pylightning/lightning/lightning.py:634: in invoice
    return self.call("invoice", payload)
contrib/pylightning/lightning/lightning.py:222: in call
    "id": self.next_id,
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <lightning.lightning.LightningRpc object at 0x7fa38c025e10>
sock = <socket.socket fd=12, family=AddressFamily.AF_UNIX, type=SocketKind.SOCK_STREAM, proto=0, raddr=lightning-rpc>
obj = {'id': 3, 'method': 'invoice', 'params': {'description': '\ud83d\udc7d', 'label': '\ud83d\udc7d', 'msatoshi': 123000}}

    def _writeobj(self, sock, obj):
        s = json.dumps(obj, ensure_ascii=False, cls=self.encoder_cls)
>       sock.sendall(bytearray(s, 'UTF-8'))
E       UnicodeEncodeError: 'utf-8' codec can't encode characters in position 63-64: surrogates not allowed

[cdecker]

@rustyrussell I added a small test with some emojis and it seems to me that this addresses the issue correctly.

[cdecker]

ACK 1379435