extension_header: make $isExtension work
test/rfc4475: 3.1.2.1. - test the right thing
Make optional the space between display_name and < in addr_spec.

See https://tools.ietf.org/html/rfc4475#section-3.1.1.6
Ignore extra whitespace around start-line elements

https://tools.ietf.org/html/rfc4475#section-3.1.2.9
https://tools.ietf.org/html/rfc4475#section-3.1.2.10
Ignore spaces directly inside angle brackets

See https://tools.ietf.org/html/rfc4475#section-3.1.2.14
test/rfc4475: 3.1.2.15. - test the right thing
sippers.parse: throw ParsedError if SIP-Version is not "SIP/2.0"

passes test/rfc4475 3.1.2.16
sippers.parse: Check CSeq method against Request method.

passes test/rfc4475 3.1.2.17, 3.1.2.18
helpers.sipuriBuild: serialize puts space after display_name
test/rfc4475: 3.1.2.3. - test the right thing
pass test/rfc4475 3.1.2.3.
test/rfc4475: update/comment some tests