Problem

HTMLForm was unconditionally quoting the boundary parameter in Content-Type headers:

Content-Type: multipart/form-data; boundary="MIME_boundary_A1B2C3D4"

While technically RFC-compliant, this was inconsistent with other parts of the codebase and caused issues with strict parsers like nginx-upload-module.

Root Cause

Internal inconsistency in Poco:

• MailMessage uses MediaType::toString() → MessageHeader::quote() (conditional quoting)
• HTMLForm used manual unconditional quoting

Solution

Changed HTMLForm to use MessageHeader::quote(), which only adds quotes when the value contains special characters. Since MultipartWriter::createBoundary() generates boundaries like MIME_boundary_A1B2C3D4E5F6G7H8 (alphanumeric + underscore only), quotes are no longer added.

RFC 2046 Compliance

Per RFC 2046 Section 5.1, quotes are only required when boundary contains non-alphanumeric characters other than ., _, -.

Benefits

1. Consistency - HTMLForm now behaves like MailMessage and MediaType
2. Interoperability - Works with strict parsers that don't handle unnecessary quotes
3. Cleaner output - Generates minimal, RFC-compliant headers

Replaces #4953