Handle more encoding conversion edge cases

http/client/response.go

http/client/response_test.go

http/client/testdata/HTTP-charset.html

http/client/testdata/HTTP-vs-UTF-8-BOM.html

http/client/testdata/HTTP-vs-meta-charset.html

http/client/testdata/HTTP-vs-meta-content.html

http/client/testdata/No-encoding-declaration.html

http/client/testdata/README

http/client/testdata/UTF-16BE-BOM.html

http/client/testdata/UTF-16LE-BOM.html

http/client/testdata/UTF-8-BOM-vs-meta-charset.html

http/client/testdata/UTF-8-BOM-vs-meta-content.html

http/client/testdata/charset-content-type-xml-iso88591.xml

http/client/testdata/content-type-only-win-8859-1.xml

http/client/testdata/gb2312.html

http/client/testdata/meta-charset-attribute.html

http/client/testdata/meta-content-attribute.html

http/client/testdata/rdf_utf8.xml

http/client/testdata/urdu.xml

http/client/testdata/windows_1251.html