Normalise Request and Response to_* methods.

**BREAKING CHANGE**

The Response#to_xml method will now return the XML::Document object the
Response is wrapping. To obtain a string representation of the Response
please use the #to_s method which accepts same arguments as Request and
XML::Document.

The Response#to_xml method would previously return an indented XML
string, to obtain the same result you must now use #to_s(indent: true).
Change current version number to 0.1.0.

This has been done as an indication of breaking changes between the
current code base and the previous 0.4.0 release.