Quick fix for RestClient

Hi,

Thank you for this useful gem !

I had to make a small patch to make it work with RestClient::Request

request = RestClient::Request.new(...)
::ApiAuth.sign!(reques, _id, _secret)

When calling request.execute, RestClient::Request references request.processed_headers via request.transmit. ApiAuth modifies headers but not processed_headers.

The small change I added made it work for me, but I admit I had to do it fast and did not look much for alternative solutions.

Best,
Nico