Allow `ExecuteRequest` to support `Accept: application/*+json`

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Is your feature request related to a problem? Please describe.

Currently the accept header is forced to XML for ExecuteRequest() and other methods, but some API (like the /cloudapi) only accepts json

go-vcloud-director/govcd/api.go

Lines 558 to 560 in 8dbe510

	func (client *Client) ExecuteRequest(pathURL, requestType, contentType, errorMessage string, payload, out interface{}) (*http.Response, error) {
	return client.executeRequest(pathURL, requestType, contentType, errorMessage, payload, out, client.APIVersion)
	}

This is forced here:

go-vcloud-director/govcd/api.go

Line 253 in 8dbe510

req.Header.Add("Accept", "application/*+xml;version="+apiVersion)

Describe the solution you'd like

If the content-type parameter is set to something json, it should automatically change the accept to JSON instead of XML

Describe alternatives you've considered

You need to reimplement the logic yourself

Additional context

No response