Wrapping for base64 parameter b= is not specified

Currently the draft says "The b parameter contains a base64-encoded blob that contains the cryptographic signature object of the type described by t."

Examples wrap the parameter.

The wording can be taken from https://www.rfc-editor.org/rfc/rfc6376: "b= The signature data (base64; REQUIRED). Whitespace is ignored in this value and MUST be ignored when reassembling the original signature. In particular, the signing process can safely insert FWS in this value in arbitrary places to conform to line-length limits. See "Signer Actions" (Section 5) for how the signature is computed."

Whitespace after CRLF is technically part of the header, so it should be removed on the application level: https://www.rfc-editor.org/rfc/rfc5322#section-2.2.3