Allow custom headers to be specified for all api requests

[fatboystring]

Hi, thanks for the library. I have an issue where a third party woocommerce api I interact with implements caching, but I always need the most up-to-date version of the data when I call the api.

I am able to bust the cache using a custom query parameter with a timestamp but this feels hacky. The third party api does respect the cache-control header and it seems generally useful to be able to specify custom headers to send with every request.

I'm hoping you'd be open to merging and releasing the changes in this PR. Please do let me know if you would like me to make any further changes, happy to help.

Thanks in advance

[benje]

I also need this functionality and @fatboystring's solution looks great!

[HubertGL]

this could also solve the problem with the X-HTTP-Method-Override header indirectly if http methods are limited to only GET and POST for security reasons. But this feature might be easier implemented as separated feature within the __request() method to only change the init method and not each put and delete for the clients