fix a error on ruby2.0 and using gzip.

I encounterd an error same as this issue.
https://github.com/sparklemotion/mechanize/issues/275

in ruby2.0, HTTPResponse seems not possible to get original body's length with gzip.
because HTTPResponse inflate automatically now.

so test of content-length checking is skiped in my patch. 



finally, i'm sorry my bad english.