Allow setting of custom *http.Client

[someone1]

Due to various reasons, the http.DefaultClient used by default might not be appropriate to use. It would be nice to be able to specify what client to use for requests.

[someone1]

Ahhh, I see PR #39 addresses this but does so globally, not on a per client basis.

[rickb777]

ForceApi could include a *http.Client value that is initialised in Create to http.DefaultClient.

type ForceApi struct {
	... existing
	httpClient *http.Client
}

Then change func (forceApi *ForceApi) request(...) error { to use the specified client instead of http.DefaultClient.

Then add a setup method

// WithHttpClient sets a specialised HTTP client to replace the default client for HTTP requests.
func (forceApi *ForceApi) WithHttpClient(c *http.Client) *ForceApi {
	forceApi.httpClient = c
	return forceApi
}

[rickb777]

btw this is a good idea, IMO.