Use custom headers in every client

[mgm1313]

What kind of change does this PR introduce?

Feature

What is the current behavior?

Since #218 custom headers passed to the SupabaseClient constructor are passed on to the GoTrueClient.
However they are not passed on to thePostgestClient, SupabaseStorageClient and SupabaseQueryBuilder, although implied by the param description:

supabase-js/src/SupabaseClient.ts

Line 45 in 5f7f3d5

* @param options.headers Any additional headers to send with each network request.

What is the new behavior?

Headers passed through the options.headers parameter of the constructor are now stored in a new protected variable headers. Since all clients mentioned above call the private method _getAuthHeaders I decided to inject the custom headers here, together with the default headers that were already injected inside this method.

Additional context

I also created a very minimal test case, just to be sure the _getAuthHeaders method indeed returns the custom headers. I'm not an expert on Jest, so as far as convention goes there might be more optimal ways to test the functionality this PR introduces.
Closes #166

[soedirgo]

Thanks @mgm1313!