Add Content-Length header

[amandasaffer]

To prevent Shopify from throwing 411 Length Required error.

[amandasaffer]

@joshrps Please accept this? Issue discussed here: #33

[eliranm]

@joshrps Can you accept this issue? It do solve the reported problem.

[xewl]

src/RocketCode/Shopify/API.php has yet to contain a default header of Content-Length: 0.
This mostly occurs on empty POST requests, like on:
order/[order_id]/fulfilments/[fulfilment_id]/complete.json