fix #53

[yeoleobun]

• Removing to tag for CANCEL, according RFC 3261 9.1 Client Behavior

The Request-URI, Call-ID, To, the numeric part of CSeq, and From header
fields in the CANCEL request MUST be identical to those in the
request being cancelled, including tags.

That means, the CANCEL should not have TO tag.

• add api get_client_dialog_by_call_id