interest: Correcting encoding/decoding of Nonce field

Nonce field is now encoded as 4-byte uint8_t value, as defined by
NDN-TLV spec.  In addition to that, setNonce will not reset
wireEncoding, rather than just modify the existing wire encoding, if it
is already exists.

Change-Id: Ibec3737ccfba70fec5c0531487782a46fbc5600a
Refs: #1600