Clean up the checks for valid SIP requests and responses; do the tests -

Clean up the checks for valid SIP requests and responses; do the tests -
and the finding of tokens in the header - only once.

Don't assume there's either zero or one spaces after the colon in a
Name:Value header, and don't put a tvb_format_text()-formatted version
of the value into the protocol tree.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@7811 f5534014-38df-0310-8fa8-9805f1628bb7