xmldocument::save: use encoding interpreted by get_write_encoding in buffered_writer constructor

[aral-matrix]

This is a preemptive patch, for something that is currently not a bug, but could become one in the future.

xml_document::save currently constructs buffered_writer with parameter encoding:
https://github.com/zeux/pugixml/blob/master/src/pugixml.cpp#L7459

The xml_buffered_writer constructor then invokes get_write_encoding to interpret that user_encoding:
https://github.com/zeux/pugixml/blob/master/src/pugixml.cpp#L3753

get_write_encoding can modify the value of encoding (that is subsequently used to initialize buffered_writer.encoding)
https://github.com/zeux/pugixml/blob/master/src/pugixml.cpp#L3618

xml_document::save proceeds to perform two checks on the unmodified local function parameter encoding:
https://github.com/zeux/pugixml/blob/master/src/pugixml.cpp#L7463 (encoding != encoding_latin1)
and
https://github.com/zeux/pugixml/blob/master/src/pugixml.cpp#L7477 (encoding == encoding_latin1)

This implementation could cause a bug in the future, if either of the following becomes true:

1. encoding_latin1 is modified to a different encoding in get_write_encoding
2. any other encoding is modified to encoding_latin1
3. a similar check is added in xml_document::save on any other encoding that is translated either to a different value or from a different value in get_write_encoding, meaning that the check would then act on the non-interpreted function parameter, possibly triggering an action for the wrong encoding

This patch implements the two checks on the interpreted buffered_writer.encoding member variable instead of the unmodified function parameter encoding.