Honor multipart boundary from an explicit Content-Type header

[Synvoya]

Description

When a multipart boundary is supplied only via an explicit Content-Type header (without the --boundary flag), HTTPie declares that boundary in the header but writes the body with a different, random boundary — so the declared boundary never appears in the body and the request is unparseable.

$ http --offline --multipart POST example.org hello=world \
    'Content-Type:multipart/form-data; boundary=xoxo'
# header says boundary=xoxo, but the body delimiter is --<random>

This is the workflow requested in #199 (setting the boundary the way curl does).

Fix

If no --boundary was given, read the boundary from the Content-Type header (via the existing parse_content_type_header, which is case-insensitive and quote-stripping, so Boundary="..." works too) and use it for the encoder. --boundary still takes precedence, so the explicit-override behavior is unchanged.

Test

Added test_multipart_boundary_from_content_type_without_flag. Full tests/test_uploads.py passes (28); reverting only the fix fails the new test.