Corrected encoding to be standard spelling.

The test was failing as it expected encoding="utf8" but the actual response was encoding="UTF-8", which is the correct spelling.

However, although this fixes the tests, I had a look and it looks like the relevant PHPunit tests isn't actually running on travis e.g. https://travis-ci.org/guzzle/guzzle/jobs/8939161 

"sh: 1: \~/.nvm/nvm.sh run v0.6.14: not found
[testserver] could not start node server"

So presumably all the tests that involve mocks through Node.js are being skipped.