Switch namespace separator from colon to pipe (fixes #289)

[hartwork]

Fixes #289

[exxamalte]

It should probably be mentioned that this fix does indeed break all parsers that use this library and that parse XML with multiple namespaces, and that have so far relied on the default namespace separator.

[hartwork]

@exxamalte that's a fair point

[sktzofrenic]

It should probably be mentioned that this fix does indeed break all parsers that use this library and that parse XML with multiple namespaces, and that have so far relied on the default namespace separator.

Leaving it as is breaks many more projects though.

Thank you @hartwork for doing the legwork on this.

[hartwork]

Closing in favor of libexpat/libexpat#577 .