signature_method_rejected on private applications

[andyplak]

I've been successfully using this XeroBundle for over a year now, but have encountered an issue when recently updating this and guzzle. When trying to put a banktransaction into Xero I get the following 400 Bad Request:

response: oauth_problem=signature_method_rejected&oauth_problem_advice=Private applications must use the RSA-SHA1 signature

After a bit of digging I came across this commit with Guzzle's oauth subscriber:
guzzle/oauth-subscriber@4736b9a

I see the you set the signature_method here, but I can't see it used anywhere.

Modifying the initialization of $middlewhere gets me up and running again:

$middleware = new Oauth1([
    'consumer_key' => $config['consumer_key'],
    'consumer_secret' => $config['consumer_secret'],
    'token' => $config['token'],
    'token_secret' => $config['token_secret'],
    'signature_method' => $config['signature_method'],
    'private_key_file' => $config['private_key'],
    'private_key_passphrase' => null
]);

Obviously that won't work for people with a passphrase set on their private key file, but its a step in the right direction.

[andyplak]

I've just spotted that pull request #7 submitted by @mradcliffe handles both this and the another issue I ran into with the trailing slash. Would be good to see that merged in.