Remove line wrapping in headers

Removes line wrapping from envelope headers. Somewhere along the way, the multi-line-ness was lost, so that by the time the calling client's conn.fetch() returned, only the first line of the header value was there.

Two effects were observed with line-wrapped subjects in the envelope, in the app that counted on matching the subject for threading goodness: The subject got chopped initially if it arrived line-wrapped; and in responses where "RE: " was added to the subject, the subject was occasionally wrapped in a different way, which caused the responses to appear as a different subject.