Removing processinginstruction node if it exists.

Ran into an issue on IE9 when processing an XML response wher the firstChild node would always be the processing instruction. (i.e. `<?xml version="1.0" encoding="UTF-8" ?>`) This simply removes it from the response if it exists and returns the body which is consistent with the behaviour in other modern browsers (tested in Chrome 33.0.1750.152).