merge revision(s) 42930: [Backport #8898]

	* lib/uri/generic.rb (URI::Generic.find_proxy): return nil if
	  http_proxy environment variable is empty string.
	  [ruby-core:57140] [Bug #8898]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@43062 b2dd03c8-39d4-4d8f-98ff-823fe69b080e