sdformat pushes visual plugin comments inside `<sdf></sdf>` tags

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/168)) by Piyush Khandelwal (Bitbucket: [piyushk](https://bitbucket.org/%7B0aafe920-14b3-4b46-927d-421c6f4e90f2%7D/)).**

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I have a sdf file that contains a visual plugin. As per this thread (http://answers.gazebosim.org/question/13494/warning-and-fail-to-read-an-sdf-element-converting-a-deprecated-sdf-sourcedata-string/), sdformat pushes the contents of the visual plugin insided `` tags.

Here's an example:
```

example

...
```
gets converted to:
```

example


...
```

It then complains about the self-inserted sdf tag with a spurious warning:
```
Warning [parser.cc:437] Converting a deprecated SDF source[data-string].
```

I don't fully understand the justification for pushing the contents into an tag, but in any case, the warning should be eliminated.