More client refactoring and cleanup

[jmigueprieto]

Follow up of #159

Changes in this PR

• Refactor code to set Accept and Content-Type headers.

[jmigueprieto]

This always return an empty contentType

// to determine the Content-Type header
contentTypes := []string{}
// set Content-Type header
contentType := selectHeaderContentType(contentTypes)
if contentType != "" {
	headerParams["Content-Type"] = contentType
}

So it doesn't really make sense.

And this is just setting to headerParams["Content-Type"] ="application/json"

// to determine the Content-Type header
contentTypes := []string{"application/json"}

// set Content-Type header
contentType := selectHeaderContentType(contentTypes)
if contentType != "" {
	headerParams["Content-Type"] = contentType
}

[jmigueprieto]

And a similar thing happens with

// to determine the Accept header
headerAccepts := []string{"application/json"}

// set Accept header
headerAccept := selectHeaderAccept(headerAccepts)
if headerAccept != "" {
	headerParams["Accept"] = headerAccept
}