update headers for BIP70 payment request

Summary:
Badger Cash and RaiPay use "ecash-" instead of "bitcoincash-" in the headers.
I'm also going to apply the same change to Electrum ABC https://github.com/Bitcoin-ABC/ElectrumABC/pull/213

Test Plan:

• Generate a payment URI: curl -L -H 'Content-Type: application/json' -d '{"outputs":[{"address":"1NLcNpAaBBMekgBZk7NxwdxwtSUTfTV8Aq","amount":560}],"currency":"ecash"}' https://pay.badger.cash/create_invoice
• Run src/qt/bitcoin-qt
• Menu File > Open URI
• Paste the "ecash:?r=<url>" (replace <url> with the url in the reply to the curl command, e.g. https://pay.badger.cash/i/DTKP4)
• Send

Repeat with the be.cash API:
curl -L -H 'Content-Type: application/json' -d '{"values":{"network":"XEC","outputs":[{"value":3000,"scriptHex":"76a914154205358162a6f59457d83dea828a15261aaba388ac"}]}}' https://p.be.cash/create-invoice

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11543