Fix content type for requests that contain a body

[lenvanessen]

When you use postOfferExport() you'll end up with an error that states

Picqer\BolRetailerV10\Exception\ResponseException
The supplied content-type media type is not supported.

This is due to an issue where the Client uses the wrong Content-Type header. This API should contain the header
application/vnd.retailer.v9+json but the current code ignores the provided produces option and defaults to application/vnd.retailer.v10+json, causing issues with some endpoints.

This change addresses this issue.

[robvanaarle]

Good find. This is introduced in v10, because v10 is only partly specified in the OpenAPI specs, so it was merged with v9.

However, I suspect your solution could break other methods. Client::postProductLabels for example produces a application/vnd.retailer.v9+pdf, so this will also be send as Content-Type header, but we're not sending data in PDF format to Bol.

The real problem lies deeper. The OpenAPI specifies both produces and consumes. The first should be (and is) sent as Accept header, the latter as Content-Type. But currently for Content-Type the constant API_CONTENT_TYPE_JSON is used, which works only when all Bol methods are for the same API version.

[robvanaarle]

@lenvanessen I opened #34 with a fix with the approach I described. Does this indeed solve your issue?