Completed and passed all header block tests.
Add support for ToHeader tests in parser_test.go

Add the infrastructure for the tests.
No tests currently written!
Return an error on multiple to/from headers.

Disallow SIP messages with multiple to: headers or multiple from:
headers.
Ignore separator chars within quotes and <,>

Respect quote- and bracket- escaping.
Don't panic on empty header bodies.
Added all to: header unit tests.
Add from: header tests.

... by copy-pasting the 'from' tests.
I feel dirty.
Add some Contact: header tests.

Still need to add some additional tests to thoroughly test multiple contact
addresses, and also to add support for "Contact: *".
Add support for "Contact: *" in parser.

Support wildcard contact headers.
Add relevant unit tests.