Remove possible existing BOM characters from XML String before passing i...

...t to `WSDL._fromXML()` and parsing it.

This PR uses the `strip-bom` module to remove possibly existing BOM characters of the `XML` string **before** it is passed to `WSDL._fromXML()` so that the `sax-parser` in `WSDL._parse` can operate on a *clean* `XML` and also `WSDL.xml` is set to the BOM-stripped String representation.

I added no tests as the `strip-bom` module is used in the way it is showed at his own repository (https://github.com/sindresorhus/strip-bom) and is well-tested there. Any tests under `node-soap` would be redundant in my opinion.