RFC 7578 fixes

[abotalov]

• Support creation of arbitrary form-data parts with Content-Type (fixes Setting Content-Type is not supported on non-file parts #5)
• Support creation of file parts without filename (fixes Creation of file parts without filename is not supported #6)
• Alias :mime_type as :content_type (because a value of Content-Type header may have charset)

[abotalov]

I think this is ready for review now.
cc @ixti, @tarcieri

By the way, was support for Ruby 2.0.0 removed intentionally in #8? http gem depends on this library.

[ixti]

yeah. I removed 2.0.0 support intentionally, but i'm fine to have it reintroduce - till we will remove that from http gem as well. 2.0.0 is unmaintaned anymore, and thus I don't see point in supporting it either.

[tarcieri]

Let's keep it for now, but officially drop support for < 2.2 after 2.1 is EOLed (which should be in February)

[ixti]

Works for me. 👍

[ixti]

@abotalov i'll provide my review commments a bit alter today

[ixti]

After taking a look. Almost everything looks good to me - I'm gonna merge and add some fixes.

[ixti]

And only now I recalled that actually string parts were supported before this PR (although not very obvious):

string_io = StringIO.new('{"foo":"bar"}')
HTTP::FormData.create({
  :foobar => HTTP::FormData::File.new(string_io, :filename => "foobar.json")
})

[ixti]

In any case I think that Part.new(str) looks a bit more cleaner, and brings us a bit closer to start work on #1