Add support for parsing XML nil values xsi:nil="true" inside elements with the xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" attribute

[VadimKulagin]

Original issue: #1
I made the necessary changes and updated the tests.

[coveralls]

Pull Request Test Coverage Report for Build 7

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 94.444%

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Totals
Change from base Build 5:	0.04%
Covered Lines:	136
Relevant Lines:	144

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💛 - Coveralls

[danilobuerger]

Hi @VadimKulagin sorry for taking such a long time to get to this. Could you please explain shortly the changes you made to the tests and the intention behind it?

[VadimKulagin]

Hi @danilobuerger.

Initially, I did not want to change tests so much, but I had to do it because their initial implementation did not allow me to perform a value check, which can have two views in XML at once.

For example, the value of nil in XML can be represented simply as <Value xsi:nil="true"></Value>, or maybe <ValueXmlWrapper xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Value xsi:nil="true"></Value></ValueXmlWrapper>.

The initial implementation of the tests did not allow testing the second option, because each test ​​had a hard format:

type nullTest struct {
	new       func() nullTestValue
	jsonValue nullTestValue
	json      string
	textValue nullTestValue
	text      string
	xmlValue  nullTestValue
	xml       string
}

Therefore, I had to make a separate test for each type of values, in which I can specify the initial and expected value.

type nullTest struct {
	Type        testType
	New         func() interface{} // Generator of *nullTestValue (and their wrappers)
	Unmarshaled interface{}
	Marshaled   string
}

And now I can specify different input and output data for the test.

	tests := []nullTest{
		{testTypeXml, int64NullValue, int64NullValue(), `<Int64 xsi:nil="true"></Int64>`},
		{
			testTypeXml,
			newInt64NullValueXmlWrapper,
			newInt64NullValueXmlWrapper(),
			`<nullTestValueXmlWrapper xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Value xsi:nil="true"></Value></nullTestValueXmlWrapper>`,
		},
	}

Thus, it became possible to test the conversion to XML and from XML with and without namespace.

[VadimKulagin]

Hello! What do you think about this pull request?

If my changes look too big, then I can do another pull request, in which there will be a separate test only for my case with XML.