gh-128110: Fix rfc2047 whitespace handling in email parser address headers (#130749)

RFC 2047 Section 6.2 requires that "any 'linear-white-space' that
separates a pair of adjacent 'encoded-word's is ignored." The modern
header value parser correctly implements that for unstructured headers,
but had missed a case in structured headers. This could cause a parsed
address header to include extraneous spaces in a display-name.

Switch to @bitdancer's fix from review feedback. Recharacterize space
between ews as fws after parsing in get_phrase.

RDM: This fix is dependent on the fact that "subsequent" atoms will never have
leading whitespace because that's been consumed already. I don't think
it's worth adding extra code for the possibility of leading whitespace
because the parser won't produce it. It's a bit of parser fragility in the
face of code changes, but I think that's a minor concern given the 
parser design (which is that it consumes whitespace greedily)

Co-authored-by: R David Murray <rdmurray@bitdance.com>