improve xml encoding

[dwagin]

Added ability to define header and body params as:

	soap.HeaderParams = gosoap.SliceParams{
		xml.StartElement{
			Name: xml.Name{
				Space: "auth",
				Local: "Login",
			},
		},
		"user",
		xml.EndElement{
			Name: xml.Name{
				Space: "auth",
				Local: "Login",
			},
		},
		xml.StartElement{
			Name: xml.Name{
				Space: "auth",
				Local: "Password",
			},
		},
		"P@ssw0rd",
		xml.EndElement{
			Name: xml.Name{
				Space: "auth",
				Local: "Password",
			},
		},
	}

Also fix #37 and partial #46