ensure potential FilePart responds to original_filename

Some gems modify String to include a content_type method. They generally
do not add an original_filename, however, and this causes problems when
creating a new FilePart
refactor Parts::Part to encapsulate file detection logic
refute -> assert !
Merge branch 'master' of github.com:steved555/multipart-post into pr-20

Conflicts:
	Gemfile.lock
	lib/parts.rb