pass api_key as Authorization header

[bishwahang]

• set api_key on Authorization headers by passing it as mandatory argument like braze_url
• re-record all the VCR request with new requests

As of May 2020, Braze has changed how we read API keys to be more secure. Now API keys must be passed as a request header. Braze will continue to support the api_key being passed through the request body and URL parameters, but will eventually be sunset. Please update your API calls accordingly.

[bishwahang]

@morgoth @jboltz88 I am passing the api_key to the base class initializers BrazeRuby::Rest::Base(https://github.com/jboltz88/braze_ruby/pull/15/files#diff-86ef3181d045a2527662b11eb98267a2R10), and also initializing the connection with Authorization header already set on BrazeRuby::Http(https://github.com/jboltz88/braze_ruby/pull/15/files#diff-4341681b37f214570d457ea93537d194R31).

The message signature has only been changed on internal methods, and the public api still remains the same. I have also re-recorded the whole VCR cassettes with new request and the specs passes.

[morgoth]

@bishwahang I released 0.4.0 with this change