Fixed #2205

Hello.
I made some fixes to `get_unicode_from_response` fixing #2205.
It now tries every encodings from meta tags by calling `get_encodings_from_content`.
Because encoding charset is case-insensitive by RFC 2616, encoding charset is appended to `tried_encodings` in lower case.

Just to be clear, I did some timings: https://gist.github.com/sigmavirus24/32c7c07b1d724c4458ea