XmlPoke always writes the xmldeclaration even if the original file didn't have one

[Roemer]

As the title says, when using XmlPoke, it always writes the declaration.

There are two possibilities:

1. Add an OmitXmlDeclaration flag to XmlPokeSettings to define if the declaration should be written or not.
2. Check in XmlPoke if the original one has a declaration and either write it or not.

Which one should we prefer?

[gep13]

@Roemer of the two options, I think I prefer the second one.

@cake-build/cake-team ?

[Roemer]

I crated a PR with solution 2.