Accept content-encoding identity

[MrChrisW]

identity is a valid content-encoding header value https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Encoding

Fixes #385

[MrChrisW]

Looks like there's legacy references to identity in the enable_gzip function

mechanize/lib/mechanize/http/agent.rb

Lines 492 to 497 in 1c79ac2

	def enable_gzip request
	request['accept-encoding'] = if @gzip_enabled
	'gzip,deflate,identity'
	else
	'identity'
	end

[knu]

Thanks!