adding a new member filePartFileSize to FilePart.
adding a comment.
removing lookup with IndexedHeader.
refactoring for addContentRange.
refactoring addContentLength.
extracting the same logic: sendResponseNoBody
PartialFileSpec.hs -> ResponseSpec.hs

Following the common practice.
If a target file name is <file>, its spec file should be <fileSpec>.
checking also Content-Range in tests.
doc update.