Allow DoRequest to take a custom content type

[nstuart-at-salesforce]

This library works great!

I'm looking to use this library to perform Metadata API deploys via the REST API. Ideally I'd use sf.DoRequest(...), unfortunately I need to set Content-Type to something custom like multipart/form-data; boundary=--------------------------BOUNDARY ----------------------------BOUNDARY, except it has a hardcoded content: jsonType in the requestPayload. By creating a new method DoRequestCustom() or something similar, it would allow expanding the use cases of this library.

I'm looking at contributing to the repo now, but wanted to file an issue to see if there were thoughts on the usefulness of this.

[k-capehart]

Hey @nstuart-at-salesforce , thanks for using go-salesforce!

That's a really good point. I think you're on to something with DoRequestCustom(). The more I think about it, the more I think it should have been like that from the beginning. I'll probably consider deprecating DoRequest in favor of whatever you come up with here. Ideally it lets you set both Content-Type and Accept headers.

In the meantime, having that extra method would be great. Thanks again!