Previously we allowed only File and StringIO objects as an input to
FormData::File, but we can generalize that to any IO object that
responds to #read and #size (which includes Tempfile,
ActionDispatch::Http::UploadedFile etc).

That way different operating systems won't attempt to convert newline
characters to their internal representation, instead the file content
will always be retrieved byte-for-byte as is.

Previously Pathname was implicitly supported, though extracting filename
wasn't working. With the recent refactoring this stopped working, so we
make the Pathname support explicit.

By changing all components to use an IO object as a base, we can
implement a common IO interface for all components, which delegates to
the underlying IO object.

This enables streaming multipart data into the request body, avoiding
loading the whole multipart data into memory when File parts are backed
by File objects.

See httprb/http#409 for the new streaming API.

By delegating handling strings to CompositeIO we can remove a lot of the
StringIO.new clutter when instantiating CompositeIO objects.

We increase it for FormData::CompositeIO#read.

This way we're not creating a new string for each chunk read, instead
each chunk will be read into an existing string object (a "buffer"),
replacing any previous content.