Add MIME::MediaType

[straight-shoota]

This PR adds a MIME::MediaType type with methods for parsing mime media types as per RFC 2045 and RFC 2046 including support for optional parameters.

This type is put to use for reading and writing HTTP Content-Type headers and HTTP::Multipart boundaries replacing existing incomplete implementations.

This is not directly integrated with the MIME registry (added in #5765) but can be optionally used together.

Closes #5686

[RX14]

Needs code comments on the "tricky bits", especially the rfc2231 handling

[Sija]

Why? Having access to raw params Hash could come handy, when removed there's no way to do that.

[straight-shoota]

MediaType directly exposes #[], #[]?, #fetch, #[]= and #each_parameter. Why would yo need direct access to params?

[Sija]

For serialization as an example.

[straight-shoota]

What kind of serialization? I figure you'd either serialize with #to_s or directly access the ivar @params.

[straight-shoota]

@RX14 I removed sort from #to_s without an different implementation for #inspect. This can be improved later, if necessary.

[RX14]

I'd still like many more code comments to explain the implicit state while parsing. Especially the rfc2231 stuff.

[sdogruyol]

Finally, thank you @straight-shoota 👍

[RX14]

Squash please!