hapic_data.files should handle always same type of file

[inkhey]

Currently:

• hapic_data.files return different type of file for each framework (cgi.FieldStorage for pyramid, FileField for aiohttp, FileUpload for bottle, FileStorage for flask, ...)
• Pyramid with serpyco input file validation is broken because cgi.FieldStorage cannot be converted as bool (complicated to check nullability if file type are all differents).

Expected:

• Return always same type of file with original file as parameters.
• Pyramid/Serpyco input files validation should work.