Fix indentation issue with SelfClosingElement

[atrico-go]

Use of SelfClosingElement gets the indentation wrong
This is because EndElement handles this but it is not done for StartElement.
Very tiny change to fix this - call indent method after SelfClosingElement has been written.

[ydnar]

Thanks! Can you add a test for this?

[atrico-go]

Sorry, I missed your comment!
I've added the test and confirmed that it fails without the fix and passes with
(I do usually follow the TDD paradigm but in this case, my test was in my own (external) code so it didn't get added to the PR)

[Integralist]

I've pulled the PR locally and run the tests both with and without the fix and I'm seeing what I would expect.

Here's the failing test when you remove the fix:

go test -v -run TestSelfClosingIndentation ./...
=== RUN   TestSelfClosingIndentation
    xml_test.go:2061: output mismatch:
        have: "<RootElement>\n   <Child1/>\n      <Child2/></RootElement>"
        want: "<RootElement>\n   <Child1/>\n   <Child2/>\n</RootElement>"
--- FAIL: TestSelfClosingIndentation (0.00s)
FAIL
FAIL    github.com/nbio/xml     0.559s
FAIL

Notice the indentation is doubled when there is no fix in place.

[cee-dub]

Notice the indentation is doubled when there is no fix in place.

And there's a newline missing.