[feat] added ability to opt-out from mime formatted header names

[aricart]

added an optional argument that allows an application's use of headers to opt-out of mime header formatting. To retrieve values, they must also opt-out

[aricart]

Implemented as specified here: #147

[aricart]

nats.deno is going to implement it as described in #147.

• set() will write header as per the user
• get() will return the first value in the first key that matches in a case insensitive way
• values() return the union of all keys that match case-insensitively.

No key normalization, headers are insensitive, if the user wants to store a canonical header, they can use the provided function to transform the key.

[wallyqs]

LGTM

[aricart]

@wallyqs @ColinSullivan1

All options with the exception of append, have an exact option which will constrain any operation to be case-sensitive
only. This optional option (false by default) will:

• set(), only replaces an exact match header
• get(), only returns an exact match header
• values() only returns an exact match header

append doesn't provide this setting, as any append is case-preserving, so only adds to an exact match.

This should honor what the user wants while providing a mechanism to further clamp for more constrained requirements.

[kozlovic]

LGTM, minor comment about performing canonicalMIMEHeaderKey() twice on a set?

[kozlovic]

LGTM

[aricart]

@kozlovic raised the point that append what preserving the input's case, but options provided could be in disagreement.

[kozlovic]

LGTM