message.el bug: message-field-value issue with folded headers

[Yuri D'Elia]

In a recent change in message.el the headers are folded automatically
before being sent.

If the header has no whitespace, it will essentially move the entire
value into the next line:

|Header:
| long-value

Does it make sense to do so?

Anyway, `message-fetch-field' doesn't handle this case correctly. By
substituting "\n[\t ]+" => " " we're left with a leading space:

  " long value"

Can we strip leading and trailing whitespace from value at the end of
the function?