Validate content-length header.

Scenario: The server returns bad content-length.
RFC 2616 says Content-Length header's format is:

   Content-Length    = "Content-Length" ":" 1*DIGIT

But some server returns invalid format.
It makes mysterious error message by Perl interpreter.

Then, Furl validates content-length header before processing.

ref. https://www.ietf.org/rfc/rfc2616.txt

Closes https://github.com/tokuhirom/Furl/issues/75#issuecomment-56290533