Interest: Make getMustBeFresh() return true if the internal answerOriginKind has not been specified. Refs #1334.
encoding: When encoding/decoding the TLV Interest MustBeFresh, when answerOriginKind is not specified, MustBeFresh is default true. Refs #1334.
tests: In test-encode-decode-interest, for freshInterest, test with setMustBeFresh(false) because the default is now true.