Feature/composite properties and authentication

[Fale]

This is further work I did on top of #4

This does:

0. Does everything the other PR does
1. Successfully execute all test
2. Support []map[string]interface{}
3. More "pluggable"

[codecov-io]

Codecov Report

Merging #8 into master will decrease coverage by 0.73%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   71.26%   70.52%   -0.74%     
==========================================
  Files           3        3              
  Lines         174      190      +16     
==========================================
+ Hits          124      134      +10     
- Misses         47       52       +5     
- Partials        3        4       +1

Impacted Files	Coverage Δ
soap.go	84.61% <33.33%> (-3.39%)	⬇️
encode.go	62.79% <80.95%> (+1.05%)	⬆️

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[Fale]

Just adding here some info about map[string]interface{} and []map[string]interface{} that should probably become an example/test, but I'm not really sure how, since I don't have a public endpoint for those.

    params := gosoap.Params{
        "String": "value",
        "Test": map[string]interface{}{
            "Test2": "value2",
        },  
        "Booking": []map[string]interface{}{
            map[string]interface{}{
                "Booking": map[string]interface{}{
                    "Day":  "asd",
                    "Room": "lol",
                },  
            },  
        },  
    }   

Becomes:

            <Booking>
                <Booking>
                    <Day>asd</Day>
                    <Room>lol</Room>
                </Booking>
            </Booking>
            <Test>
                <Test2>value2</Test2>
            </Test>
            <String>value</String>

[chaz8080]

@Fale Maybe I have a fringe case, but is it possible to add attributes to params as well as values? See AccountInfo e.g.

params := gosoap.Params{
    "Booking": []map[string]interface{}{
        map[string]interface{}{
            "Booking": map[string]interface{}{
                "AccountInfo": map[string]interface{}{
                    `xml:"num,attr"`: "12345",
                    `xml:"secret,attr"`: "topsecret",
                    `xml:"country,attr"`: "US",
                    `xml:"laungauge,attr"`: "en",
                },
                "Day":  "asd",
                "Room": "lol",
            },  
        },  
    },  
} 

<Booking>
    <Booking>
        <AccountInfo num="12345" secret="topsecret" country="US" language="en" />
        <Day>asd</Day>
        <Room>lol</Room>
    </Booking>
</Booking>

Might also be related to #23