Symbolize method in RequestSignature

This fixes an issue I had with Webmock today, exemplified in the spec I added to `spec/acceptance/typhoeus/typhoeus_hydra_spec.rb `.

Initializing a `Typhoeus::Request` with http method as a string wasn't matched correctly and Webmock kindly gave me instructions to add a `stub_request(:post, ...)` with method as symbol. This should fix the issue and ensure that we symbolize http method attribute in `RequestSignature` so no one encounters this issue again.