Change test so we make use of new reset functionality in SimpleHttpResponseProvider.

This allows us to set up MockHttpServer only once instead of for each
test. Makes test run faster.
Don't use deprecated HTTP.UTF_8 constant anymore.
Update MockHttpServer so Content-Type of response is only set when non empty + only write content when content is provided in response.
Update test to reset response code to default value at end of test.
Add test that checks that providing response with null Content-Type and Content works.
Update documentation. Add section on 1.2 release.