Enable option to skip base64 encoding of headers with pure ASCII characters

[samwan-loops]

Problem: Spam detection servers such as rspamd consider certain Base64-encoded headers to be red flags, and increases the spam score of emails with such headers. By default, MIMEText will applies base64 encoding to all headers, which means that emails encoded with the MIMEText library will have higher spam scores by default.

Proposal: Provide an optional flag in EnvironmentContext to skip base64 encoding for a header if it contains only ascii characters. Since this flag is optional, there is no breaking change.

Approach:

• Add an optional skipEncodingPureAsciiHeaders flag to EnvironmentContext to skip encoding headers with pure ASCII values.
• Add a helper function optionallySkipPureAsciiEncoding to MIMEMessageHeader to detect whether a value is pure ASCII, and skip encoding it if the option is enabled.
• Use this helper function in headers that use dumpMailboxMulti or dumpMailboxSingle, and for the Subject header.
• Update the MIMEMessageHeader.spec.js unit test to cover encoding headers with non-ascii characters, and optionally encoding headers with ascii-only characters.

Comment: There are some unit tests not passing, but they are not passing in the main branch and do not seem to be due to the changes in this PR.